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Tlie liemistry
o
by Ced ric L Ch ernick
U S ATOMIC ENERGY COMMISSION
Division of Technical Information
Understanding
the tom
eries
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Tlie Cliem istryof
tlie nolDle gases
by Cedric L Che rnick
CONTENTS
THE GASES THEMSELVES 1
Discovery 2
Occurrence and Production
4
Uses 7
EARLY HISTORY 10
Attempts To Form Compounds
10
Why the Gases Are Inert 11
PREPARATION OF THE FIRST XENON COMPOUNDS 18
COMPOUNDS OF XENON
23
Fiuorme Contammg Compounds 23
Oxygen Contammg Compounds
28
More Complex Compounds
31
COMPOUNDS OF OTHER NOBLE GASES 32
Rddon 32
Krypton
32
Helium Neon and Argon 33
SHAPES OF MOLECULES 33
Solid State
33
Gas Phase
35
Predicted Shapes and Chemical Bonding 37
POSSIBLE USES 43
SUGGESTED REFERENCES 45
Uni ted States A to m ic Energy Comm ission
Div is ion of Technica l In format ion
Library of Congress Catalog Card Nu m ber 67 629 72
1967
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These luminous Geisler tube script signs were
made by E. O Sperling, a glassblower at the Na-
tional Bureau of Standards, for the 1904 Louisi-
ana Purch ase Exposition, St. Louts, Missouri
They are believed to have been the first exam-
ples of the use of the noble gases and hydrogen )
for display purposes. Each tube was filled by
P. G. Nutting, an NB S scientist, with a sam-
ple of the appropriate gas obtained directly from
Sir William Ram say see page J). About 1930,
the commercial use of neon tube signs began see
page 7), and since then neon signs have become
comm onplace the world over. Meanw hile, until
1962, at least, the noble gases rema ined among
the mo st fascinating, m.ost puzzling, and least
known of all elements.
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, - l i . .
I - ^ I I 11 ^ ll
1
in
Tlie CHemistry
of
tlie nolDle gases
B y C E D R I C
L
C H E R N I C K
THE GASES THEMSELVES
If you've made upyour mind that ch em istry is adull
subject , and want tocontinuetothink so, you shouldnot
read this booklet . It will only upse t your com fortable
con
viction. If that should happen, itwill
be
quite tradit ional ,
by
the way,
be cau se information about
the
"noble g as es "
has been shat ter ing cherished bel iefs with remarkable
consis tency
for
some year s
now.
For over 6 y e a r s the 6 ga ses helium, neon, argon,
krypton, xenon,
and
radon were
the
confirmed bac he lors
among
the
known elements .
All the
other elements would
enter into chemical combination withone oranotherof their
kind, i r respect ive ofwhether they w ere sol ids , ga ses
,
or
l iquids
in
the i r normal s ta te .
Not so
helium, neon, argon,
krypton, xenon, andrad on . They w ere ch em ically aloof
and
would have nothing
to do
with other elements ,
or
even with
one another.
This behavior earned them aunique po sitionin the Pe-
riodic Tableof
the
E lement s
and
they were called names
like
the
" i ne r t ga s e s "
orthe
"noble gases".* They were
also labeled the " ra re ga se s" , a lthough helium and
argon
a re
not
r e al ly " r a r e " . t
The inability
of
these gases
to
form chem ical compounds
was ,
unti l 1962, oneofthe most accepted fundamentals in
* " N o b l e " byr e a s o n of t h e i r a p p a r e n t r e l
u c t a n c e to mingle wi th
t h e c o m m o n h e r d of e l e m e n t s .
t X e n o n , h o w e v e r ,
is
the r a r e s t of all s t a b l e e l e m e n t s on e a r t h
.
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E l e m e n t
Hel ium
Neon
Argon
Krypton
Xenon
Radon
Symb
He
Ne
A r
K r
Xe
Rn
c h e m i s t r y . T h e n a l o n g c a m e s o m e s c i e n
t i s t s w i th w h a t
P h i l i p A b e l s o n , e d i t o r of t h e m a g a z i n e
Science, l a t e r c a l l e d
" a g e r m of s k e p t i c i s m " . In t h e s p a c e of o n
l y a c o u p l e of
m o n t h s a l l t h e d o g m a r e l a t i n g t o t h e i n
e r t n e s s of x e n o n w a s
o v e r t h r o w n i t h a d d e f i n i t e l y b e c o m e a
" j o i n e r " . R a d o n
a n d k r y p t o n b e g a n " m i n g l i n g " c h e m i c a
l l y s o o n t h e r e a f t e r
a n d , a l t h o u g h th e o t h e r t h r e e g a s e s a r e
s t i l l h o l d i n g o u t , t h e
d a m a g e t o a f i r m l y c h e r i s h e d b e l i e f w a
s d o n e .
Table I
ABUNDANCE OF NOBLE GASES IN AIR AT SEA LEVEL
Par t s per Mi l l ion (by volume)
5
18
9430
1
0.1
6 X 10~'^
S o m e i d e a of t h e e x c i t e m e n t t h e s e d i s c o
v e r i e s c a u s e d
a m o n g s c i e n t i s t s c a n b e g l e a n e d f r o m t
h e f a c t t h a t , l e s s
t h a n a y e a r a f t e r t h e f i r s t d i s c o v e r y of
a x e n o n c o m p o u n d
w a s a n n o u n c e d , a c o n f e r e n c e on " N o b l e G
a s C o m p o u n d s "
w a s h e l d a t A r g o n n e N a t i o n a l L a b o r a t o
r y n e a r C h i c a g o .
S o m e 1 0 0 s c i e n t i s t s d i s c u s s e d w o r k t h
e y h a d d o n e i n t h e
f ie ld , a n d a l m o s t 60 m a d e f o r m a l r e p o r t s
T h e p r o c e e d i n g s
of t h a t m e e t i n g f i l l e d a 4 0 0 - p a g e b o o k e
n t i t l e d Noble Gas
Compounds.
* N o t b a d , c o n s i d e r i n g t h a t j u s t a s h o r
t t i m e
b e f o r e n o t even one n o b l e g a s c o m p o u n d w a s
k n o w n .
T h i s b o o k l e t w i l l a t t e m p t to s h o w h o w t h
e s e g a s e s l o s t
t h e i r b a c h e l o r h o o d , a n d w h y t o d a y t h e
y a r e c a l l e d " h e l i u m
g r o u p g a s e s " o r " n o b l e g a s e s " i n s t e a d
of " i n e r t g a s e s " .
Discovery
T h e f i r s t i n d i c a t i o n of t h e e x i s t e n c e
of a n i n e r t c o n s t i t u
e n t i n t h e a t m o s p h e r e c a m e in 1 7 85 w h e n H
e n r y C a v e n d i s h t
fo u nd th a t h e c o u l d n o t c o n v e r t a t m o s p h e
r i c n i t r o g e n c o m -
*Edi t ed by H . H . Hym an . See Sugges t ed R ef e r en ce s ,
page 45 .
t T h e g r e a t E n g l i s h c h e m i s t a nd p h y s i c i
s t w h o a l s o d i s c o v e r e d
h y d r o g e n .
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pletely to nitrous acid. He concluded that, "if there is any
part of our atmosphere which differs from the rest . . . i t
is
not m or e than 1/120 p a rt of the whole". Th is re su lt
was
apparently forgotten or neglected, and the problem arose
again in stu die s on the density of nitrog en in the ea rly
1890s. At that time Lord Rayleigh* discovered that nitrogen
obtained by removal of the then known gases from an air
sample, or "atmospheric ni t rogen", was denser than ni t
ro-
Sir William Ram say
gen prep ared by chemical m ea ns tha t i s , "chemical n i t
ro
gen". A num ber of th eo rie s we re advanced for the d is c re
p
ancy in the densities of the nitrogen samples from the two
so ur ce s. E i ther the "ch em ical" ni trogen was too l ight ,
or
the "atm osph eric" ni t rogen too heavy, becau se of the pr e
s
ence of other gases. In 1894, however. Lord Rayleigh and
W ill iam R am say t showed that the "atm os ph eric " ni t
rogen
was a m ixture of nitrogen and a hea vier, previou sly un di
s
co ve red , ga s. This gas turn ed out to be a new elem ent
that
was given the name "argon", on account of its chemical
inactivity (from the Greek word, argon, meaning inactive,
idle).
*John W. S t ru t t , w ho inh e r i t e d the t i t l e Lo rd R
ay le igh , w a s d i
r e c t o r of t h e C a v e n d i s h L a b o r a t o r y a t C
a m b r i d g e U n i v e r s i t y in
Eng land w hen he d id th i s im po r tan t w ork . H e i s a lm
os t a lw a ys
r e f e r r e d t o b y h i s t i t l e .
tR am sa y w a s a Sco t s c he m is t w ho w as kn igh ted in
1902 . H e r e
ce iv ed the 1904 N obe l P r i z e in ch em is t ry fo r h i s
d i sc ov er i e s of
nob le g a se s . Lord R ay le igh r ec e iv ed the 1904 N obe l
P r i z e in
phys ics in r ecogn i t ion o f h i s n i t rogen s tud ies w i
th R am say .
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The d i s co ve ry of t he o th e r 5 g as es fo ll owed rap id
ly ; by
1900 they had a l l be en i so l a t e d and i den t i f i ed .
Ra m sa y and
h i s a s s i s t a n t , M o r r i s T r a v e r s , in c o nt
in u in g t h e i r r e s e a r c h
on a rgo n mad e us e of newly deve loped me tho ds fo r l i que
fy
i ng g a s e s . T h e e a r t h ' s a t m o s p h e r e c o n s
i s t s m a i n ly of n i t r o
gen (78%), oxy gen (21%), and ar go n (1%), w hich hav e bo i l
ing
p o i n t s s u ff ic i e n tl y d i f f e r e n t ( - 1 9 5 . 8
C , - 1 8 2 . 9 6 C , a nd
1 8 5 . 7 C , r e s p e c t i v e l y ) t h a t t h e y c a n r
e a d i l y b e s e p a
ra t e d by f r ac t i on a l d i s t i l l a t i on of l i qu
id a i r . As R am sa y
a n d T r a v e r s i m p r o v e d t h e i r t e c h n i q u e
s , t h e y fo un d t h a t
t h e y c o u l d o b t a in s e v e r a l m o r e f r a c t i o
n s w h en d i s t i l l i n g
l iq u id a i r . T h r e e of t h e s e f r a c t i o n s c o n
t a i n e d e l e m e n t s
n e v e r b e f o r e i s o l a t e d , n a m e l y , n e o n (
G r e e k ,
neos,
new),
k r y p t o n ( G r e e k ,
kryptos,
h idde n) , and xenon (G reek ,
xenon,
s t r a n g e r ) .
R a m s a y w a s a l s o i n s t r u m e n t a l in d i s c o v
e r i n g t h e e x i s
t ence o f he l i um (Greek , helios, t he sun ) . Th i s e l e
m en t had
bee n no t ed i n t he su n ' s sp ec t ru m as ea r ly a s 1868
, bu t was
o nly i s o l a t e d a s a t e r r e s t r i a l e l e m e n t
i n 1 89 5 w h e n R a m s a y
o b t a in e d i t b y h e a t i n g th e u r a n i u m - c o n
t a i n i n g m i n e r a l c l e v e -
i t e . * ( T he h e l i u m i n t h i s m i n e r a l w a s p h
y s i c a l l y t r a p p e d
and was no t chemica l l y combined . )
The f ina l nob l e ga s t o be d i sc ov er ed wa s r a do n .
In 1900
F r i e d r i c h D o r n , a G e r m a n p h y s i c i s t , fo
un d t h a t r a d i u m
e v o l v ed a g a s t h a t h e c a l l e d " r a d i u m e m a
n a t i o n " . T h i s g a s
w a s l a t e r g i v e n t h e n a m e niton, bu t s in ce 1923
i t h a s be en
known as
radon.
A l l i s o t o p e s of r a d o n a r e r a d i o a c t i v e
.
Occurrence and Production
T h e a t m o s p h e r e i s o u r m a j o r s o u r c e f o r
n e o n , a r g o n ,
k ryp ton , and xenon , and t h es e g a se s a r e now p ro duc
ed
c o m m e r c i a l l y a s a b y - p r o d u c t d u r i n g f
r a c t i o n a l d i s t i l l a ti o n
of l iqu id a i r t o p ro du ce l i qu id oxygen and n i t r og
en . L iqu e
fac t i on of t hous and s o f t ons o f a i r p e r day m ak e
s t h es e 4
g a s e s a v a i l a b l e i n s u f f i c i e n t q u a n t i
t i e s f o r p r e s e n t n e e d s .
He l ium i s t he s ec ond m o s t ab undan t e l em en t i n t
he u n i
v e r s e . A b o ut 7 6% of t h e m a s s of t h e u n i v e r
s e , i t i s e s t i -
*Th i s m ine ra l i s a l so know n a s u r an in i t e ; one
va r i e ty of u r a n -
i n i t e ,
p i t c h b l e n d e , i s a n i m p o r t a n t s o u r c e of
u r a n i u m f o r p r o d u c
t ion of a tomic energy .
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m ated, is hyd rogen; helium m ak es up about 23%, and all
the
oth er elem en ts tog ethe r co m pose the re m ain ing 1% of
the
m a s s .
H elium is so light tha t it is continually esc ap ing from
the ear th ' s a tmosphere into inters te l lar space. The
present
concentration of helium in the atmosphere therefore prob
ably represents a steady-state concentration, that is , the
amount being re le ase d f rom the ea r t h ' s c ru st i s
equal to
the amount e scaping from the atm os ph ere into sp ac e.
The
con stant esc ape exp lains why t he re is so little to be
found
in our air. Helium can be obtained from the atmosphere in
the sam e way neon, argo n, krypton , and xenon a re , but
is
m or e readil y obtained from accu m ulation s that have
built
up in the ear th ' s crust .
Th is helium in the e art h is continually being formed by
radioactive decay. All radioactive materials that decay by
emitting alpha particles produce helium, since an alpha
particle is nothing more than a helium nucleus with a posi
tive charge. Most of the helium in the earth 's crust comes
from the decay of uranium and thorium.
The helium is obtained by tapping natural gas wells,
which yield an ave rag e h elium con tent of about 2%. M ost
of thes e helium we lls a re in an ar ea within 250 m ile s
of
A m aril lo, T ex as , although s m all am ounts have been
found
in natural gas elsewhere in the U. S. Since the early 1950s
helium-containing gases also have been found in South
A frica, R us sia , and Can ada. In oth er p a rt s of the world
the
helium content of natural gases and mineral springs is too
low to make separat ion commercia l ly a t t ract ive .
The helium is recovered from the natural gas by an ini
tia l liquefaction tha t lea ve s only heliu m and nitro ge n
in
gaseous form. Fu r the r l iquefaction, this t ime under p r e
s
s u r e ,
ca us es mo st of the nitrog en to condense and leav es
helium of about 98% pu rity in the gas p h as e. Th is can
be
fur ther purified by pass in g it through a l iq ui d- ni tro ge
n-
cooled trap containing charcoal, which absorbs the re
maining impuri t ies .
The final one of our noble gases, radon, is obtained from
the radioactive decay of radium. One gram of radium pro
duc es about 0.0001 m il li li te r of radon p er day. (We
should
* F o r m o r e a b o u t r a d i o a c t i v i t y s e e Our
Atomic World and o ther
book le t s i n t h i s s e r i e s .
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a J ,(> =l -.
Figure 1
A U. S. Bureau of Mines helium plant in Keyes, Okla-
hom a, uitli the cold boxes ', or refrigerating units, in the
fore-
ground
ke ep in mind , how eve r , t ha t 1 g r am of r a d i um i s a
ve ry
l a rg e amo un t i n t e r m s of t he t o t a l ava i l ab l e
.* ) Radon ha s a
s h o r t h a l f -l i f e ( th e c o m m o n e s t i so t op e
,+ c o m i n g f r o m r a
d ium , i s r ad on -22 2 who se ha l f - l i f e i s 3.8 da ys
) , wh ich
m ea ns t ha t abou t ha lf t he r ad on a to m s wi l l d i s i
n t eg ra t e
in a l i t t l e u nd er 4 da ys . S ince r ad ium ha s a m uch
long er
ha l f - l i fe than tha t , about 1620 y e a r s , the am oun t
of dau gh ter
r a d o n i n c o n t a c t w it h t h e p a r e n t r a d i u m
r e a c h e s a c o n s t a n t
concen t ra t i on . In o the r words t he amoun t o f r adon be
ing
p r o d u c e d i s b a l a n c e d b y t h e a m o u n t d i s
i n t e g r a t i n g , a nd a s
*F ro m the d i s c ov e ry of r ad ium by M ar i e and P i e r
r e Cu r i e in
1898 un t i l 1940 on ly about 1000 g ra m s we re i so la ted ,
and a l though
pro du c t io n in cr ea se d du r ing W or ld W ar I I, i t i s
doubt fu l w he ther
th e r e a r e m or e t han 100 g r a m s of pu re r ad ium ava
i l ab l e in t he
W es t e rn W or ld t oday .
t I so topes a r e t he va r iou s fo rm s of t he s a m e e l
em en t . Fo r a f ull
de f in it i on of t h i s and o th e r un fam i l i a r w o rds
, s e e Nuclear Terms,
A Brief Glossary, a com pan ion book le t in t h i s s e r i e s
.
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soon as the pr im ary so ur ce (the radium) is rem ove d,
the
radon concentration begins to decrease because of i ts con
tinuin g d isi nt eg rat io n. After 1 half-life (3.8 days) only
half
the radon re m ai ns ; after a second half- life,
%
of that will
have disintegrated, that is
%
of
%
o r
%;
in a month there
will be le s s than 1% left; and after
n
half-lives the fraction
remaining wil l be
{%) .
The amou nt of rado n one can
iso lat e at any given tim e is , th er ef or e, d ependent on
the
amount of radium originally available.
A number of isotopes of the noble gases can be produced
artif icially, either directly by bombardment in a particle
a cc el er at o r, or as the prod uct of decay of an
artificially
excited atom, or by nuclear
fission.
The la tte r method is
used for production of krypton and xenon in atom ic re ac to rs
.
Fi ssio n is a pro ce ss in which a heavy atom spli ts to
form
2 ligh ter atoms of approximately equal m as s* ; one or
m or e ne utro ns and a la rg e amount of energy also are
r e leased s imul taneous ly .
Uses
Many of the uses of these gases are outgrowths of their
in er tn es s . The gre ate r abundances , and hence lower cos
ts ,
of helium and argon re su lt in their use as in er t atm o
spheres in which to weld and fabricate metals . The elec
tr ical and other properties of the noble gases make most
of them ideal gases for filling numerous types of electronic
tubes and in lasers. For this , the gases may be used singly
or mixed with one or m ore of the ot he rs. Pe rh ap s the be
st
known use is in the fam iliar "neon " adv ertisin g sign s.
The
glow produced by neon alone is red. The other gases pro
duce less bril l iant colors: helium (pale pink), argon (blue)
,
krypton (pale blue), and xenon, (blue-green).
H elium, be cau se of its li gh tn es s, finds us e a s a
lifting
gas for balloo ns and a ir sh ip s, although it is he av ier
than
hydrogen. This weight d isadvan tage, how ever, is far o ve
r
balanced by the fact that helium is nonflammable. Recently,
* F o r e x a m p l e , if u r a n i u m - 2 3 5 f i s s i o n s
, k r y p t o n - 9 0 a n d b a r i u m -
144 ,
o r x e n o n - 1 4 0 a n d s t r o n t i u m - 9 4 m i g h t b
e f o r m e d .
t F o r a f ull exp l ana t i on of f i s s i on , s e e Our
Atomic
World
a
c o m p a n i o n b o o kl e t i n t h i s s e r i e s .
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5
C
M
3
D
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Figure 3
A technician checks a liquid-helium refrigerator prior
to shipment. This unit is designed to cool masers and
supercon
ducting magnets used for space comm unication.
h e l iu m h as b een u s ed a s a co o l in g m ed iu m in n u
c l ea r r eac
t o r s , and i t i s a l so a d i luen t fo r oxygen in b r ea
th in g s y s
t e m s f or d e e p - s e a d i v e r s . H e l i u m b e i n g
l e s s s o l u b le i n t h e
b lo o d t h a n n i t r o g e n , t h e h e l i u m - o x y g e
n m i x t u r e i s p r e f e r
a b le t o n o r m a l a i r f or p e r s o n s w o r k i n g u
n d e r p r e s s u r e ,
s i n c e i t s u s e t e n d s to p r e v e n t t h e b e n d s
, a s e r i o u s c o n
d i t ion cau sed by gas bu bb les in the body f lu ids and t i
s su e s .
L iqu id he l iu m , which i s the on ly su bs tan ce tha t w i
ll r em a in
l iq u i d a t t e m p e r a t u r e s c l o s e t o a b s o l u
t e z e r o ( - 2 7 3 C ) , i s
f in d in g i n c r e a s i n g u s e in l o w - t e m p e r a t
u r e p h y s i c s cryo
genics.* R ad o n h a s b een u s ed a s a s o u r c e of g am m
a r a y s
f o r t r e a t m e n t o f c a n c e r , b u t m o r e c o n v
e n i e n t g a m m a - r a y
s o u r c e s p ro d u c e d in n u c l e a r r e a c t o r s no
w a r e m o r e f r e
q u en t ly ch o s en f o r m ed ic a l t h e r a p y .
*See Cryogenics, Tlie Uncom mon Cold another booklet in this
se r ies , fo r an explana t ion of th i s b ranch of sc ience
.
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EARLY HISTORY
Attempts To Form Compounds
As in the ca se of o t he r e l e m en ts , the d i s co ve ry
of the
nob le g a se s wa s fo llowed by an exa m ina t io n of th e i
r ch em
i c a l p r o p e r t i e s . It s o on b e c a m e o b vi o us
t h a t t h e s e e l e m e n t s
w e r e d i f f e r e n t t h e y w o u ld n ot e n t e r i n to
c h e m i c a l c o m
b ina t ion wi th any o th e r e l em en ts o r wi th one an o
th e r . M any
a t t e m p t s w e r e m a d e to i n d u c e c h e m i c a l r
e a c t i o n s b e t w e e n
n o b l e g a s e s a nd b o t h m e t a l s a nd n o n m e t a
l s . A g r e a t m a n y
t e c h n i q u e s w e r e u s e d b u t n o ne p r o v e d s u
c c e s s f u l . A l t ho u g h
m a n y c l a i m s w e r e m a d e t h a t c o m p o u n d s h
a d b e e n f o r m e d
con ta in ing nob le gas a t om s che m ica l ly bound to o th e
r
a t o m s , m o s t of t h e s e e i t h e r w e r e u n c o n v
i n c i n g o r s h o w n
t o b e i n c o r r e c t . T h e s c i e n t i s t s w ho c a m
e c l o s e s t t o s u c c e s s
w e r e t h e A m e r i c a n c h e m i s t s , D o n Y o s t a
n d A l b e r t K a y e .
In 1933 they s e t ou t to t e s t the p r ed ic t ion , m ad e
tha t ye a r
b y a n o t h e r A m e r i c a n , L i n u s P a u l i n g , t
h a t k r y p t o n a nd x e n on
m i g h t r e a c t w i th f l u o r i n e . Y o s t an d K a y
e p a s s e d e l e c t r i c
d i s c h a rg e s th ro ug h m ix tu re s of xenon and f luo r
ine and of
k r y p t o n a nd f l u o r i n e . T h e i r r e s u l t s w e
r e i n c o n c l u s i v e a nd
t h e y s t a t e d i n a c o m m u n i c a t i o n t o t h e
Journal of tlie Am eri
can Chemical Society, I t can no t be sa id tha t de f in i t e
e v i
den ce for com pound for m at io n w as found . It do es no t fo
l low,
o f cou r se , tha t xenon f luo r ide i s incapab le o f ex i s
t ing .
V ery soon a f t e r the d i sc ov e r y of the nob le ga se s i
t w as
shown tha t a rgo n , k ry p to n , and xenon wi l l fo rm hy dr
a t e s
c o m p o u n d s i n w h i c h t h e g a s e s a r e a s s o c
i a t e d w i th w a t e r
m o l e c u l e s . A t f i r s t t h e h y d r a t e s w e r e
th o u g h t t o b e t r u e
c h e m i c a l c o m p o u n d s , b u t t h e y w e r e l a t
e r s h o w n t o b e
clathrate com poun ds ; in th i s type of compound the in e r t
gas
i s t r a p p e d i n h o l e s i n a c r y s t a l l i n e c a
g e f o r m e d b y t h e
w a t e r m o l e c u l e s . T h e h o s t m o l e c u l e in h
y d r a t e s i s w a t e r ,
b u t s e v e r a l o t h e r c l a t h r a t e h o s t s ha v e
a l s o b e e n u s e d , s u c h
as the o rg an ic com pou nds pheno l and qu ino l . F o r a c o
m
p o un d t o a c t a s a h o s t t h e c a v i t i e s i n i t s
c r y s t a l l i n e s t r u c
t u r e m u s t b e l a r g e e n o u g h t o p r o v i d e r o
o m f o r t h e i n e r t
gas a tom , bu t sm a l l enough to kee p i t t r a pp ed in the
cag e .
So f a r no hos t m o l ec u le s have been found who se ca ge s
a r e
sm a l l enough to ke ep he l ium o r neon a to m s t r a pp ed
, so no
10
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7/23/2019 The Ch
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c l a t h r a t e c o m p o u n d s of t h e s e g a s e s a r e
k n own . In c i d e n t a l l y ,
t h e p h e n o m e n o n of c l a t h r a t e fo rm a t i o n p
ro v i d e s a me t h o d
of se pa ra t in g neon f rom a rg on by t ra pp ing the a rgo n
in a
c l a th ra t e cage and pum ping off t he neon .
C l a t h r a t e c o m p o u n d s a r e n ot t r u e c h e m i
c a l c o m p o u n d s ,
b e c a u s e t h e y d o no t c o n t a i n r e a l c h e m i c
a l b o n d s . T h e o n ly
fo rc e s b e t w e e n t h e i n e r t g a s a n d t h e h o s
t m o l e c u l e a r e
r e l a t i v e l y w e a k e l e c t r o s t a t i c i n t e r
a c t i o n s . T h e i n e r t g a s i s
r e a d i l y r e l e a s e d b y d e s t r o y i n g t he c r y
s t a l l i n e c a g e , e i t h e r
by d i s so lv in g the ho s t i n a su i t ab le so lven t o r
by hea t ing i t
t o i t s me l t ing po in t .
Why the Gases Are Inert
B e fo re d i s c u s s i n g t h e r e a s o n s fo r t h e i n
e r t n e s s of t h e
nob le g as es i t i s i n t e re s t in g to look a t t he re l
a t io ns h i ps
b e t w e e n e l e m e n t s , a n d h ow t h e y c o m b i n e
c h e m i c a l l y w i th
one an o th e r . The the o ry tha t each e l e m en t ha s a f
ixed co m
b i n i n g c a p a c i t y wa s p r o p o s e d by t h e E n g
l i s h c h e m i s t S i r
E d w a rd F ra n k l a n d i n 1 8 5 2 . T h i s c a p a c i t
y w a s c a l l e d th e
valence of an a to m . As m os t of t he e l em en t s then
known
would co m bine wi th e i th e r oxygen o r hyd roge n , t he va
le nc e
va lue s w er e re l a t e d to the num be r of a tom s of
oxygen o r
hy dro gen wi th w h ich one a to m of each e l e m en t wou ld
c o m
b i n e .
Tw o a to m s of hy dro ge n c om bin e wi th 1 a to m of
oxygen
to for m H2O, so hyd rog en w as g iven a va len ce of 1 , and
ox y
gen a va len ce of 2 . T he va len ce of any o th er e le m en t
w as
then the number o f a toms o f hydrogen (o r twice the
number
of oxygen a to m s) tha t com bin ed w i th 1 a tom of tha t e l
em en t .
In am m on ia we have the fo rm ula NH3, so n i t ro ge n ha s
a
va len ce of 3 ; in ca rb on d iox ide , CO2, the ca rb on va
len ce i s
4 .
V a l e n c e s a r e a l w a y s w ho le n u m b e r s . S o m
e e l e m e n t s
e x h i b i t mo re t h a n o n e v a l e n c e , a n d t h e ma
x i mu m v a l e n c e
appea rs to be 8 .
In t h e l a t e 1 8 6 0s t h e R u s s i a n c h e m i s t D m
i t r i M e n d e l e e v
ma d e a n i n t r i g u i n g o b s e rv a t i o n wh e n l i s
t i n g t h e e l e me n t s
in the o r d e r of i n c r ea s i ng a tom ic we ig h t s . He
found tha t t he
f i r s t e l em en t a f t e r hyd roge n w as l i t h ium wi
th a va len ce of
1, the s eco nd h eav ies t w as be ry l l iu m wi th a va len
ce of 2 ,
the th i r d , bo ro n wi th a va len ce of 3 , and so on . As
he c on
t inued he found a sequence of valences that went 1 , 2 , 3 , 4
,
-
7/23/2019 The Ch
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3 , 2 , 1 , and then repea ted itself If h e a r r a n g e d t h
e e l e m e n t s
in v e r t i c a l co lu m ns nex t t o one an o th e r , i n t
he o rd e r of
i n c r e a s i n g a t o m i c w e i g h t s , h e fo un d t h
e e l e m e n t s i n e a c h
h o r i z o n t a l r o w a c r o s s t h e p a g e h a d t h e
s a m e v a l e n c e a n d
s t r i k i n g l y s i m i l a r c h e m i c a l p r o p e r t
i e s .
T h i s k in d of p e r i o d i c i t y , o r r e g u l a r r e
c u r r e n c e , h ad b e e n
n o t e d b y o t h e r s c i e n t i s t s , b u t M e n d e l
e e v m a d e a g r e a t s t e p
Dmitri Mendeleev
fo rw a rd by l e av ing ga ps in h i s t ab l e w he re the nex
t known
ele m en t , in or d e r of we igh t , d id not f it b ec au se
i t had the
w r o n g v a l e n c e o r t h e w r o n g p r o p e r t i e s
. H e p r e d i c t e d t h a t
t h e s e g a p s w o u ld b e f il le d b y y e t - t o - b e -
d i s c o v e r e d e l e
m e n t s , a nd h e e v e n w e n t a s f a r a s t o p r e d i
c t t h e p r o p e r t i e s
of so m e of t h e s e e l em en t s f rom the po s i t i on
they wou ld
occupy in h i s t ab l e . A re p r od uc t io n of an ea r ly
ve rs io n of
M e n d e l e e v ' s P e r i o d i c T a b l e of t h e E l e m
e n t s i s s h o w n i n
F i g u r e 4 . A s c a n b e s e e n , t h i s w a s b a s e d
o n t h e 6 3 e l e m e n t s
t h e n k n o w n . I n l a t e r v e r s i o n s of t h e T a b
l e t h e e l e m e n t s a r e
a r r a n g e d in o r d e r a c r o s s t h e h o r i z o n t a
l r o w s , a nd t h o s e
w i th s i m i l a r p r o p e r t i e s f al l in t h e s a m e
v e r t i c a l c o lu m n .
At the t im e of t he se t t i ng up of t he Pe r io d ic T ab
le t he
n o b l e g a s e s w e r e s t i l l u n d i s c o v e r e d .
T h e r e w e r e n o g a p s
l e ft fo r t he m , a s sp a c e s cou ld be l e ft on ly w he
re a t l e a s t 1
e l e m e n t i n a g r o u p w a s a l r e a d y k n o w n . W
h e n a r g o n w a s
d i s c o v e r e d s o m e p r o b l e m t h e r e f o r e a r
o s e a s to i t s p l a c e
12
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Figure 4
Above is an early (1869) version of Men deleev's Periodic
Table. The heading reads, Tentative system of the elements .
The subheading reads, Based on atomic Heights and chemical
similarities . This table is reprod uced from D m it r i
Ivanovich
M e n d e l e e v , N. A. Figurovskii, Russian Academ y of
Science, Mo s
cow, 1961.
i n t h e p e r i o d i c s y s t e m . I t s a t o mi c w e i g
h t s u g g e s t e d i t m i g h t
b e l o n g s o m e w h e r e n e a r p o t a s s i u m . W h en
i t s l ac k of c h e m
i c a l r e a c t i v i t y w a s d i s c o v e re d , M e n d e
l e e v p ro p o s e d t h a t i t
had ze ro va len ce and shou ld co m e be tw een ch lo r ine
and
p o t a s s i u m . He s u g g e s t e d t h a t a g ro u p of s
u c h g a s e s mi g h t
be found. Th e va le nc e p er io d ic i t y the n would be 0 ,
1 , 2 , 3 ,
4,
3 , 2 , 1 . T his new gro up led to a co m ple te pe r io d i c
i ty
of 8 , wh ich we sh a l l se e i s a v er y s ign i f ic an t nu
m be r .
B o th F ra n k l a n d a nd M e n d e l e e v b a s e d t h e i
r i d e a s on t h e i r
k n o w l ed g e of c h e m i c a l p r o p e r t i e s . T h e
t h e o r e t i c a l s u p p o r t
fo r bo th p ro p os a l s ca m e wi th the dev e lop m en t of
a the o ry
of a t o m i c s t ru c t u r e an d t h e
electronic
theo ry of va l en ce .
T h e o r i e s s t a t i n g t h a t m a t t e r i s c o m p o
s e d of s m a l l , i n d i
v i s i b l e p a r t i c l e s , c a l l e d a t o m s , h ad b
e e n p r o p o s e d a s
-
7/23/2019 The Ch
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ea r ly a s 400 B .C . , bu t w e r e of a ph i loso ph ic ra th
e r t han
s c i e n t i f i c n a t u r e . T h e s c i e n t i f i c a t
o m i c t h e o r y r e a l l y
s t a r t ed wi th the Eng l i sh sc i en t i s t John Da l ton
in t he 19 th
c e n t u r y . In h i s t h e o r y s m a l l , i n d i v i s i
b l e , a n d i n d e s t r u c t i b l e
p a r t i c l e s a l s o w e r e c a l l e d a t o m s , b u t
h e g a v e t h e m p r o p
e r t i e s t h a t h a d p h y s i c a l s i g n i f i c a n c
e . M o r e i m p o r t a n t ,
D a l t o n ' s t h e o r y n o t o n ly w o u ld e x p l a in o
b s e r v e d e x p e r i
m e n t a l r e s u l t s , b u t a l s o c o u ld p r e d i c t
t h e r e s u l t s of n e w
e x p e r i m e n t s .
T ow ard the end of the 19th cen tu ry the d i s co v er y of
the
e l e c t r o n d e m o n s t r a t e d t h at a t o m s t h e m
s e l v e s w e r e d i v i s i b l e
a n d l e d t o t h e p r o p o s a l of t h e o r b i t a l a t
o m . T h e a t o m c a m e
to be co ns id e r ed a s be ing m ad e up of a nu c le us , con
ta in in g
m o s t of t h e m a s s , a nd e l e c t r o n s r e v o l v i
n g a r o u n d t h e n u
c l e u s r a t h e r l i k e t h e p l a n e t s r e v o l v e
a r o u n d th e s u n . * E a c h
e l ec t ro n ha s a s ing le o r un i t neg a t ive cha rg e
and the en t i r e
a t o m i s e l e c t r i c a l l y n e u t r a l , o r u n c h
a r g e d , b e c a u s e in t h e
n u c l e u s t h e r e a r e a n u m b e r of p r o t o n s ( e
q u al t o t h e n u m b e r
of e l ec t r o n s ) , e ach of wh ich ha s a un i t po s i t i
v e ch a r g e .
Atomic Num ber Th e n u m b er of p ro to ns in a g iven a to
m
of an e l emen t i s c a l l ed the atomic number. In ad di t io
n to
t h e p r o t o n s , t h e n u c l e u s c o n t a i n s u n c
h a r g e d p a r t i c l e s c a l l e d
n e u t r o n s . T h e n e u t r o n s a n d p r o t o n s h a
v e a b o u t t h e s a m e
m a s s ,
a n d t h e e l e c t r o n s , by c o m p a r i s o n , h a v e
n e g l i g i b l e
m a s s . A n e l e m e n t of a t o m i c m a s s (A) a n d a t
o m i c n u m b e r
(Z ) w i ll h a v e a n u c l e u s c o n s i s t i n g of Z p r
o t o n s a n d ( A - Z )
n e u t r o n s , a nd t h i s w i ll b e s u r r o u n d e d b
y Z e l e c t r o n s . F o r
ex am pl e , an a tom of l i t h iu m wi th m a s s (A) of 7 and
a tom ic
n um b er (Z) o f 3 wi l l have a nuc leu s c on s i s t i ng of
3 p r o
t o n s a nd 4 n e u t r o n s ( A - Z ) , s u r r o u n d e d b
y 3 e l e c t r o n s .
The l i g h te s t e l e m en t , hy dro gen , ha s Z equ a l t
o 1 , and
e a c h s u c c e s s i v e l y h e a v i e r e l e m e n t d i
f f e r s f r o m t h e o n e
p re ce d i ng i t by an in c r e a se of 1 i n Z , and ha s one
m o r e
p r o t o n a nd o n e m o r e e l e c t r o n th a n t h e n e
x t l i g h t e r o n e .
T h u s ,
t h e s e c o n d h e a v i e s t e l e m e n t , h e l i u m ,
h a s Z e q u a l
t o 2 , a n d s o o n . F o r t h e h e a v i e r e l e m e n t
s , s u c h a s u r a n i u m
(Z = 92) , one m igh t im ag ine a ch ao t i c s i t u a t io n
wi th m any
* T h i s t h e o r e t i c a l m o d e l of t h e a t o m h a s
s i n c e b e e n m o d i f ie d
to exp la in add i t iona l ex pe r im en ta l r e s u l t s m o
re fu l ly . Now an a tom
of ten i s co ns ide red a s a nu c leu s w i th e l e c t r on
s m oving rap id ly and
ran do m ly a roun d ity and ha v i r ^ no de f in i te bound
ary su r fa ce .
14
-
7/23/2019 The Ch
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e l e c t ro n s b u z z i n g a l l a ro u n d t h e n u c l e
u s . Fo r t u n a t e l y , t h e
e l e c t r o n s a r e r e s t r i c t e d t o m o v e m e n t
in c e r t a i n fix ed o r
b i t s o r
shells.*
T h e n u m b e r of e l e c t ro n s i n e a c h s h e l l , a
n d
t h e o rd e r i n wh ic h a d d i t io n a l e l e c t r o n s
b u i l d u p t h e s h e l l s
of h e a v i e r e l e m e n t s , i s g o v e r n e d b y q u a
n t u m m e c h a n i c a l
c o n s i d e r a t i o n s . T h e f i r s t s h e l l m a y c
o n t a i n 2 e l e c t r o n s ,
the sec on d one 8 , the th i rd 18 and so on . H ow ever ,
the
m a x i m u m n u m b e r of e l e c t r o n s p o s s i b l e
in a ny o u t e r m o s t
she l l i s 8 .
Su b s h e l l s T h e s h e l l s t h e ms e l v e s a r e a c
t u a l l y s p l i t i n t o
s u b s h e l l s , w h i c h a r e d e s i g n a t e d b y t h
e l e t t e r s s, p, d, a n d / ,
s u c c e s s i v e l y m o v i n g o u t w a r d f ro m t h e n
u c l e u s . T h e n u m b e r
of e l e c t ro n s i n a g i v e n s u b l e v e l i s r e s t
r i c t e d , b e i n g a
m ax im um of 2 fo r s , 6 fo r p, 10 fo r d, and 14 f o r / .
Th e
v a r i o u s s h e l l s a r e d i s t i n g u i s h e d f r o
m o n e a n o t h e r by n u m
b e r s f r o m 1 t o 7 , w h e r e 1 i n d i c a t e s t h e i
n n e r m o s t s h e l l
a nd 7 th e o u t e r m o s t . A f u r t h e r r e s t r i c t
i o n i s t h a t t h e r e i s
only an s su ble ve l for the f i r s t sh e l l , on ly s and p
for the
se co nd , and only s , p, and d for the th i rd . Beyond the th
i rd
l eve l s ,
p, d,
a nd / s u b l e v e l s a r e a l l p e r m i t t e d . T h e s
e r e
s t r i c t i o n s a r e a c t u a l l y t h e s a m e a s t h
o s e i n d i c a t e d i n t h e
p r e c e d i n g p a r a g r a p h ; n a m e l y , t h e f i r
s t s h e l l c o n t a i n s 2
e l e c t r o n s , w h ic h we w r i t e I s ^ , t h e s e c o
n d s h e l l h a s 8 ,
wri t ten 2s^2/ )^ , the th i rd 18 , wri t ten 3s^3p^3d^\
T h e e l e c t r o n s d o n o t n e c e s s a r i l y f il l t
h e s h e l l s a nd s u b -
s h e l l s i n c o n s e c u t i v e o rd e r . T h e f i r s t
( l ig h t e st ) 18 e l e
m e n t s ' e l e c t r o n s a r e a d d e d r e g u l a r l y
, t h e e l e c t r o n s f i ll in g
the I s , 2 s , 2p, 3s , and Zp s u b s h e l l s i n s e q u e
n c e . H o w e v e r ,
i n t h e n i n e t e e n t h e l e m e n t , t h e ne w e l e c
t ro n d o e s n o t g o in t o
th e 3d su bs he l l , a s m igh t be exp ec te d , bu t i n to
the 4s s u b -
s h e l l . (Qu e s t i o n s of t h i s s o r t a r e d e c i d
e d o n t h e b a s i s of
e n e r g y c o n s i d e r a t i o n s . It i s e n e r g e t i
c a l l y m o r e f a v o r a b l e
to pu t t he 19 th e l ec t ro n in to the 4s su bs he l l . ) F
ro m th i s
p o i n t o n we c a n w r i t e d own t h e e l e c t r o n i c
c o n f i g u ra t i o n s
of the suc cee d in g (heav ie r ) e l e m e n t s on ly if we
know the
*A s h e l l i s a l s o r e f e r r e d t o in o t h e r t h e
o r i e s a s a n energy level.
t Q u a n t u m m e c h a n i c s i s a f o r m of m a t h e m a
t i c a l a n a l y s i s i n v o l v
ing quan ta , o r de f in i te un i t s o f en e rg y in which r
ad i a t io n i s em i t t ed
o r a b s o r b e d . T h e d if f er e n t o r b i t s , o r e
n e r g y l e v e l s , of p l a n e t a r y
e l e c t r o n s a r e s e p a r a t e d f r o m e a c h o t h
e r b y w h o le n u m b e r s o l
q u a n t a .
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order in which the subshells are filled. We should note that
when the electrons do go into the
M
subshell this is con
sidered to be inside the 4s level. Consequently, there can
be 10 ele ctro ns in this sub shell without violating the ru
le
of having a maximum of 8 ele ctr on s in the ou term os t sh ell
.
Table II
E L E C T R O N I C S T R U C T U R E S O F S E L E C T E D E L
E M E N T S
E l e m e n t
H y d r o g e n
H e l i u m
L i t h i u m
B e r y l l i u m
B o r o n
N e o n
S o d i u m
A r g o n
P o t a s s i u m
C a l c i u m
S c a n d i u m
T i t a n i u m
A t o m i c
N u m b e r
1
2
3
4
5
1 0
1 1
18
1 9
2 0
2 1
2 2
l s
l s 2
ls\
ls\
l s 2 ,
ls\
ls\
1 S 2 ,
ls\
ls\
1 S 2 ,
ls\
E l e c t r o n i c S t r u c t u r e
2s
2s2
2s\
2s\
2s2
2s\
2s\
2s\
2s\
2s\
2/)i
2/>S
2p^, 3 s '
2 />^ 3s2, 2,p^
2p^, 3s2 , 3 / )^ 4s*
2/>6, 3s2, 3p, 4s2
2/>, 3s2, 3p\ 3d\
2p^,
3s\ 3p^,
3rf2,
4s2
4s 2
This is always the case for
d
and / su bs hel ls: they a re
always ins ide the next or n ext-b ut-o ne s subs hell when
being fi lled. Table II gives the ele ctro nic s tr u ct u re s
for
seve ra l e l ements .*
Now we a r e read y to look at the ele ctr on ic theo ry of
valen ce and som e of it s co ns eq ue nc es. About 1920 a nu
m
b er of che m ists , m ost notably the A m erican G. N. Lew
is,
suggested that the electrons in the outermost shells were
responsible for e lements ' chemical react ions. Compounds
(that is , m olecules) a re formed by the tra ns fer o r shar
ing
of electrons, and the number of such electrons provided or
obtained by an atom of any element during the combining
p ro ce ss is i t s vale nce. How ever, ther e is a kind of re g
u
lation of the num ber of elec tro ns that can pa rtic ipa te in
this
bonding. It was suggested that the elements were always
being prodded to attain the maximum number of electrons
in their outer shell , namely 8. An electronic structure
with
*F or a d i s cu s s io n of the e lec t ro n ic c on f igu ra t
ion of an o th e r
i n t e r e s t i n g f a m i ly of t h e e l e m e n t s s e
e
Rare Earths, The Fraternal
Fifteen,
a c o m p a n i o n b o o k l e t i n t h i s s e r i e s .
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8 e lec tro ns in the o uter shell is co nsid ered to be m
ore
stable and is called a closed-shell a r rangem ent . A toms
,
then, tend to adjust their electronic structure to that of
the
nearest
elem ent with a completed outer sh ell . The adju st
m ent is mad e by losing , gaining, or sh arin g ele ctro ns
with
other atoms.
The close d-s he l l arra ng em en t of ele ctro ns happens
to
be the e lectro nic st ru ct u re of ato m s of the noble g a se
s.
Moreover , only the 6 noble ga ses have this arra ng em en t
of m aximum stab il i ty. Th is fact is the b a si s for the sh
o rt
hand notat ion for wri t ing electronic s t ructures . From
Ta ble II we can see the ele ctro nic st ru c tu re of sodium
is
Is^,
2s^,
2p^,
3s ;
sodium ha s 1 elec tron m ore than the
c losed-shel l a r rangement I s^ , 2s^ , 2p^, which is the
elec
t ron ic s t ru ctu re of neon. The sodium electron ic
configura
t ion can therefore be writ ten (Ne), 3s^ Similarly potas
sium can be writ ten (Ar), 4s\ scandium can be indicated
by (Ar), 3d*, 4s^, etc. The close(3-shell arrangements are
also called
cores.
Two atoms with the same number of electrons outside
a stable co re would tend strongly to adjust the ir e le c
tro nic configuration in a si m ila r m an ne r; that is ,
they
would have the same valence and therefore the same chem
ical p ro p er t ie s. This fact is bo rne out by the fact that
e le
m en ts in the sam e group in the Per io di c Tab le have
the
sam e outer e lec t ronic s t ru c t u re s . Table III on pages
2 4 -2 5
is a m odern ve rsio n of the Pe riod ic Tab le, showing the
elec t ronic s t ruc tures . Note tha t d i f ferent e lements
some
t im es appear to have ident ical e lec t ron ic s t ru ct u re
s; for
example, the outer shells of calcium and zinc are both 4s^.
H ow ever, ca lciu m is (Ar), 4s^ wh ile zinc is (A r), 3rf", 4
s ^
The presence of the complete d sub she ll cau ses zinc to
have somewhat di fferent propert ies . Those elements in
which the
d
and / su bsh ells a re being fil led a re called
tran
sition
elements, as opposed to the
nontransition
e lements
in which the electrons are going into s and
p
subshe l l s .
The fact that the noble gases have completed outer
sh e lls m ea ns that they have nothing to gain by losin g,
gaining, or shar ing electrons. They al ready have the s
table
e lec t ronic s t ruc tures tha t o ther e lements are s t r iv
ing to
atta in. Th is m ean s that they should have zero valence
and
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s h o u l d n o t f o r m c h e m i c a l c o m p o u n d s . T
h u s , t h e o b s e r v e d
e x p e r i m e n t a l f a c t t h a t th e g a s e s w e r e i
n e r t w a s s u p p o r t e d
b y t h e o r y . T h i s s t a r t l i n g a g r e e m e n t b
e t w e e n e x p e r i m e n t a nd
t h e o r y w a s s u c c e s s f u l i n d i s c o u r a g i n
g a t t e m p t s t o m a k e
c h e m i c a l c o m p o u n d s w i th t h e n o b l e g a s e
s f o r a p e r i o d of
a l m o s t 4 0 y e a r s .
PREPARATION OF THE FIRST
XENON COMPOUNDS
Unti l 1962 a l l the accepted ev idence poin ted to the fac
t
t h a t t h e n o b l e g a s e s w e r e c h e m i c a l l y i
n e r t . A few b r a v e
s o u l s h a d p r e d i c t e d t h a t c o m p o u n d s of t
h e m m i g h t e x i s t ,
b u t t e x t b o o k s a n d t e a c h e r s s t r e s s e d t
h e i n e r t n e s s of t h e
g a s e s a n d t h e s e s t a t e m e n t s w e n t u n c h a
l l e n g e d .
As we have s een , the d i s covery o f the f i r s t nob le gas
was
an ou tcome o f an inves t iga t ion o f the dens i ty o f n i t
rogen .
Th e d i s co ve ry o f the f i r s t ch em ica l compound of a
nob le
g a s w a s a l s o a b y - p r o d u c t of a n u n r e l a t e
d i n v e s t i g a t i o n .
T h e b e g i n n i n g r e a l l y g o e s b a c k t o t h e M
a n h a t t a n P r o j e c t *
a nd t h e p r o d u c t i o n of t h e f i r s t a t o m i c b
o m b . An i m p o r t a n t
ing red ien t fo r the bo m b w as the u r an iu m i so top e
^^^U, T h i s
w a s s e p a r a t e d fr o m n a t u r a l u r a n i u m (w h
ic h i s a m i x t u r e
con ta in ing m os t ly a no the r i s o to pe , ^^^u) by gas eo
us d i f fu
s i o n , t h e "g a s " f o r t h i s p r o c e s s b e i n g a
v o l a t i l e u r a n i u m
c o m p o u nd , u r a n i u m h e x a f l u o r i d e , U F g.
T h i s w a r t i m e i n t e r
e s t i n UFg c r e a t e d a n i n t e r e s t i n o t h e r m
e t a l l i c h e x a
f l u o r i d e s , c o m p o u n d s c o n t a i n i n g 6 fl u
o r i n e a t o m s b o u nd t o
1 m e t a l a t o m . T h e st u d y of t h e p r o p e r t i e
s of t h e s e c o m
p o u n d s , a n d t h e s e a r c h f o r n e w h e x a f l u
o r i d e s , w a s u n d e r
t a k e n a f t e r t h e w a r i n m a n y l a b o r a t o r i
e s , e s p e c i a l l y t h o s e
of t h e U . S . A t o m i c E n e r g y C o m m i s s i o n ,
wh i c h h a d w o r k e r s
e x p e r i e n c e d in h a n d l in g s u c h c h e m i c a l
l y r e a c t i v e m a t e r i a l s .
A g r o u p of s c i e n t i s t s a t t h e A E C ' s Ar g o n
n e Na t i o n a l L a b
o r a t o r y w a s p a r t i c u l a r l y a c t i v e i n t h
i s f ie l d. T h e y d i s
c o v e r e d h e x a f l u o r i d e s of p l a t i n u m , t e
c h n e t i u m , r u t h e n i u m ,
a nd r h o d i u m , a n d i n v e s t i g a t e d t h e p r o p
e r t i e s o f t h e s e a n d
o t h e r h e x a f l u o r i d e m o l e c u l e s .
*The W or ld W ar II code na m e fo r the p r o g r am of the W
ar
D e p a r t m e n t u n i t t h a t p r e d a t e d th e p r e s
e n t A t o m i c E n e r g y C o m
m i s s i o n .
18
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The nex t s t ep in the s to ry took p lace a t the Univers i ty
o f
B r i t i s h C o lum bia in V anc ouv er , w he r e N e i l B a
r t l e t t , a young
B r i t i s h c h e m i s t , w a s d o in g r e s e a r c h o n
f l u o r i d e s of p l a t i -
i nu m . H e and one of h i s co l l ea gu es d i s c ov e r ed
a compoun d
con ta in ing p la t inu m , oxygen , and f luo r in e , w hich
they fo r
m ula ted a s 02"'"PtFg~. In o r d e r to form th is typ e of c
o m
pound, an e l ec t ro n m u s t be re m ov ed f rom the O2 p a r
t of the
m ole cu l e , l ea v in g i t w i th a ne t po s i t i ve ch a
r ge . T h i s e l e c t r o n
be co m es a s s o c i a t ed w i th t he P tFg p a r t , g iv
ing t h i s pa r t a
n e t n e g a t i v e c h a r g e . T h e s u r p r i s i n g t
h i n g a b o u t t h i s r e a c
t i on i s t ha t t he ene r gy r e qu i r e d t o r e m ov e an
e l e c t r o n
f r om an oxygen m ole cu l e , t he
ionization potential,
i s qu i te
h igh . A s a m a t t e r of f ac t , no compound con ta in
ing
Oz"*"
had
e v e r bee n known be fo re the d is c o v e ry of 02 '^PtFg""
. Al though
the O z '^P tFg" t hey f i r s t s yn the s i ze d w a s no t m
ad e d i r e c t l y
f r om P tFg , B a r t l e t t s oon found tha t P tFg and m ol
ec u l a r oxy
gen wi l l r eac t to g ive th i s compound. This sugges ted to
h im
tha t P tFg (p la t inum hex af luo r ide) m us t have a s t ro
n g a f
f i n i t y f o r e l ec t r ons .
Soon af te r th e d i sc o ve ry of 02"'"P tFg~ . B ar t l e t t
r e a l i z e d
tha t the io n iza t ion po ten t ia l o f xenon i s a lm os t
ex ac t ly the
s a m e as t ha t of m o l ec u l a r oxygen . T h i s l ed h im
to w onder
if t he p l a t i n um hex a f lu o r id e , w i th i t s pow e
r f u l e l e c t r o n -
a t t r a c t i n g p r o p e r t i e s , c o u l d p u l l an e
l e c t r o n a w ay fr o m
xenon to form a ch em ic a l com pou nd. He dec ided to t r y
an
ex pe r im en t t o co n f i r m th i s ide a . H e f il led a g
l a s s co n t a ine r
wi th a known am ount of the deep re d p la t inu m he xa f luo
r ide
v a p o r a n d s e p a r a t e d i t b y a g l a s s d i a p h
r a g m f ro m a s i m i l a r
co nt a i ne r f i ll ed wi th a known am oun t of the c o lo r
l es s
xenon g as . W hen the d i ap h r a gm be tw e en t hem w as b r
o ke n
t h e r e w a s a n i m m e d i a t e a nd s p e c t a c u l a r
r e a c t i o n : T h e 2
g a s e s c o m b i n e d to p r o d u c e a y e l lo w s o l i
d I n it ia l m e a
s u r e m e n t s of t h e a m o u n t s of g a s e s r e a c t
i n g i n d i c a t e d
tha t t he com bin ing r a t i o w as 1 to l. In the Ju n e
1962
Proceedings of the Chem ical Society of London,
B a r t l e t t
r e p o r t e d p r e p a r a t i o n of t h e w o r l d ' s f i
r s t c o m p o u n d i n
which a noble g as was c he m ica l ly bound the ye l low so l
id ,
X e+P tFg" .
T h e a n n o u n c e m e n t w a s g r e e t e d w i th s u r p
r i s e a nd in
s o m e p l a c e s disbelief. T h i s i s no t s u r p r i s i
n g s i nce one of
t h e a c c e p t e d a n d r e v e r e d d o g m a s of c h e m
i s t r y h a d j u s t
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b e e n s h a t t e r e d b y h i s o n e e x p e r i m e n t .
M o r e s u r p r i s e s w e r e
y e t t o c o m e .
T h e s c i e n t i s t s a t A r g o n n e , w h e r e P t Fg h
a d b e e n f i r s t
m a d e , c o n f i r m e d B a r t l e t t ' s r e s u l t s a
l m o s t a s s o o n a s t h ey
le a r ne d of h i s ex pe r i m en ts . They wen t on to ex
tend h i s
wo r k , a n d s h o w e d t h a t x e n o n wo u l d a l s o c
o m b i n e a m a z
i n g l y w i t h t h e h e x a f l u o r i d e s of p l u t o n
i u m , r u t h e n i u m , a n d
r h o d i u m .
Ho we v e r , t h i n g s w e r e n o t a s s t r a i g h t f o
r w a r d a s t h ey h a d
a t f i r s t s e e m e d . T h e c o m b i n i n g r a t i o s
, wh i c h h a d b e e n
1-to-l i n t h e f i r s t e x p e r i m e n t s , w e r e fo un
d t o v a r y i r
r e g u l a r l y f r o m o n e h e x a f l u o r id e to a n o
t h e r , a n d s o m e
t i m e s e v e n v a r i e d f o r t h e s a m e h e x a f l u
o r i d e . E v e n w i t h
P t Fg i t a p p e a r e d t h a t t h e r e m i g h t b e a t l
e a s t 2 c o m p o u n d s
fo rm ed , Xe PtFg and Xe(PtFg)2 . T h i s th re w so m e doub t
on
the id ea t ha t X ePtFg and 02 '^P tFg~ m igh t be com ple te
ly
an a lo go us . Th e g rou p a t A rgon ne beg an to w ond er if
the
a t t r a c t i o n b e t we e n x e n o n a n d t h e h e x a f
l u o r i d e s wa s d u e ,
n o t t o t h e s t r o n g a t t r a c t i o n of t h e h e x a
f l u o r i d e s f o r e l e c
t r o n s , b u t i n s t e a d t o t h e h e x a f l u o r i d
e s ' a b i l i t y to p r o v i d e
f l u o r i n e , t h a t i s , t o a c t
as fluorinating
ag en t s . If th i s w er e
s o , i t w as r eas on ed , the xenon m igh t ac tu a l ly r e
a c t wi th
f l u o r i n e itself.
Ho w a r d H . C l a a s s e n , t h e n a n Ar g o n n e c o n
s u l t a n t f r o m
W heaton C ol le ge , and H en ry Se l ig and Joh n G. M alm of
the
Ar g o n n e C h e m i s t r y D i v i s i o n n e x t d e c i d
e d t o t e s t t h i s i d e a .
A known am oun t of xenon w as c ond ensed in a n ic ke l co
n
ta in e r and a f ive fo ld e x c e s s of f luo r ine wa s add
ed . Th e
co n t a in e r w as s e a le d and he a ted to 400 C fo r 1 ho
ur . Af te r
c o o l i n g th e c o n t a i n e r t o t h e t e m p e r a t u
r e of d r y i c e ( - 7 8 C ),
t h e e x p e r i m e n t e r s p u m p e d t h e u n r e a c t
e d g a s a w a y . If
x e n o n we r e r e a l l y a n i n e r t g a s , t h e c o n t
a i n e r s h o u l d h a v e
b e e n e m p t y a t t h i s s t a g e . T o e v e r y o n e '
s s u r p r i s e i t w a s
not e m p t y wh e n we i g h e d . F u r t h e r m o r e , t h
e g a i n i n we i g h t
cou ld be a ccou n ted fo r exac t ly by as su m in g tha t a l
l the
xenon in i t i a l ly p re se n t had r ea c t ed wi th f luo r
in e to fo r m a
compound wi th the fo r m ul a XeF 4 . The c on ten t s of the
can
w a s s u b l i m e d i n to a g l a s s t u b e a s b r i l l i
a n t , c o l o r l e s s
c r y s t a l s ( F i g u r e 5 ) . W i t h in we e k s o f t h
e t i m e t h e o r i g i n a l
a n n o u n c e m e n t f t h e p r e p a r a t i o n of Xe P t
Fg r e a c h e d A r
gonn e , a s im pl e com pound co n ta in ing a nob le ga s and
one
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Figure
Crystals of xenon tetrafluoride. Also see cover photo-
graph.)
o th e r e l e m e n t h ad b e e n p re p a re d . Th e d a t e
w a s A u g u s t 2 ,
1962.
O ne m i g h t w o n d e r w hy t h e e x p r e s s i o n m o r
e s u r p r i s e s
w e re y e t t o c o m e w a s u s e d a c o u p le of p a r a g
ra p h s a g o .
T h o se who had o bje c ted to the v i o l a t io n of t l ie
ide a of
a b s o lu t e i n e r tn e s s of t h e n o b le g a s e s c o
u ld s t i l l r a t i o n a l i z e
th a t a co m poun d a s exo t ic a s one be tw ee n Xe and PtF
g
m igh t no t con ta in t r u e ch em ic a l bond ing , and tha t
i t migh t
e v e n b e a n e w ty p e of c l a t h r a t e c o m p o u n d
. Th e p re p a ra t io n
of XeF4 re m ov ed a l l such po ss ib le ex p la na t ion s ,
and the
ch em ic a l wo r ld w as faced wi th the naked t ru th tha t a
t l e a s t
o n e i n e r t g a s w a s n o t i n e r t . C h e m i c a l t
e x t b o o k s b e c a m e
o b s o le t e o v e rn ig h t i n t h i s r e s p e c t , a nd
p ro fe s s o r s a nd
t e a c h e r s h a d t o r e w r i t e t h e i r l e c t u r e
n o t e s .
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COMPOUNDS OF XEIMON
Fluorine-Containing Compounds
As we h a v e a l r e a d y s e e n , t h e f i r s t n o b l e
g a s c o m p o u n d s
c o n t a i n e d t h e e l e m e n t f l u o r i n e .* Of t h
e m a n y c o m p o u n d s
d i s c o v e r e d s i n c e t h e n , i t t u r n s o u t t h
a t t h e y a l l e i t h e r c o n
t a i n f l u o r in e o r a r e m a d e f r o m f l u o r i n e
- c o n t a i n i n g c o m
p o u n d s . L e t u s c o n s i d e r f i r s t th e 3 k n own
b i n a r y f l u o r i d e s ,
th a t i s , com po un ds con ta in ing on ly xeno n and f lu o
r i ne . By
h e a t i n g t o g e t h e r a m i x t u r e of x e n o n a n d
f l u o r i n e u n d e r
a p p r o p r i a t e c o n d i t io n s , c h e m i s t s c a n
p r o d u c e x e n o n d i -
f l u o r i d e , Xe F2 , x e n o n t e t r a f l u o r i d e ,
Xe F4 , a n d x e n o n h e x a -
f l u o r i d e , Xe Fg . W h i c h of t h e s e f l u o r i d e
s i s p r o d u c e d d e
p e n d s o n t h e r a t i o of f l u o r i n e t o x e n o n ,
t h e t e m p e r a t u r e
of t h e r e a c t i o n , a nd t h e p r e s s u r e i n t h e
r e a c t i o n v e s s e l .
T h e se m ay b e ad ju s ted to fo rm any one of the 3 f lu o r
ide s
i n a r e a s o n a b l y p u r e s t a t e . If c a r e i s n o
t t a k e n , h o w e v e r ,
m i x t u r e s of t h e f l u o r i d e s r e s u l t a n d t h
e s e a r e d i ff i cu l t t o
s e p a r a t e . T a b l e IV o n p a g e 26 s h o w s t h e c
o n d i t i o n s t h a t
h a v e b e e n u s e d t o p r e p a r e s e v e r a l - g r a
m q u a n t i t i e s of X e F 2 ,
XeFi , and XeFg .
In o r d e r t o p r e p a r e a f l u o r i d e of x e n o n i
t i s o nly n e c e s
s a r y t o h a v e a s o u r c e o f f l u o r i n e a t o m s
, wh i c h t h e n r e a c t
wi th the xen on . He a t ing f lu o r in e g a s i s one way to
p r od uc e
s u c h a t o m s ; t h e y h a v e a l s o b e e n p r o d u c
e d b y s u b j e c t i n g
f l u o r i n e , o r f l u o r i n e - c o n t a i n i n g c o
m p o u n d s , t o e l e c t r i c
d i s c h a r g e s o r i o n i z i n g r a d i a t i o n s , s
u c h a s t h e g a m m a r a y s
f r o m a c o b a l t - 6 0 s o u r c e o r a b e a m of e l e c
t r o n s , a b e a m of
u l t r a v i o l e t l ig h t , o r a b e a m of n e u t r o n
s f r o m a r e a c t o r .
T h e f a c t t h a t x e n o n f l u o r i d e s c a n b e f o
r m e d a n s w e r e d
a p u z z l i n g q u e s t i o n t h a t h a d b e e n p l a g
u i n g s c i e n t i s t s a n d
e n g i n e e r s w ho w e r e s t u d y in g r e a c t o r f u
e l s , t In e x p e r i
m e n t s t o t e s t th e fu e l s a n d f ue l a s s e m b l i
e s f o r a m o l t e n -
s a l t r e a c t o r , a m i x t u r e of l i t h i u m f l u o
r i d e , b e r y l l i u m
f l u o r i d e , z i r c o n i u m f lu o r i d e , a n d u r a
n i u m fl u o r i d e w a s
Fluorine is the most active nonmetallic element, and
combines
with all other elements (disregarding the noble gases) so
strongly
that it cannot be prep are d from any of its natu ral compounds
by
any purely chemical reduction.
tFor more about reac tors , see
Nuclear Reactors
and
Atomic
Fuel,
companion booklets in this series.
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Table I I I PERIODIC TABLE OF THE ELE ME NTS
\ G R O U P
\ .
PERIOD\
1
2
3
4
5
6
7
la
1.00797
H
Is
6.939
Li
2 .
11
22 9898
Na
3 .
15 39.102
K
4s
37 85.47
Rb
5s
132.905
Cs
Ss
(223)
Fr
6s26p67.l
l l a
I l ia
IVa
4
9 0122
Be
2.2
12
24312
Mg
3.2
2 0 40.08
Ca
4.2
38 87.62
Sr
5.2
56 .37.34
Ba
6.2
(226)
Ra
6.26p67.2
2 1 44.956
Sc
3J '4 .2
39
^ ' 88.905
Y
4
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Table IV
CONDITIONS USED FOR PREPARING THE XENON FLUORIDES
Compound
XeF2
XeF4
XeFg
Ra t io Xe /F2
7 . 5 : 1
1 :5
1:20
T e m p e r a t u r e
(C)
400
400
250
T i m e
(hours )
16
1
16
P r e s s u r e
( a t m o s p h e r e s )
75
6
50
u s e d a s t h e f u e l. T h i s w a s s e a l e d i n a c o n
t a i n e r a n d s u b
j e c t e d t o n e u t r o n i r r a d i a t i o n . U n d e r
t h e s e c o n d i t i o n s t h e
^^^U in the u r an iu m f lu o r i de , UF4 , u n d e rg o e s f
i s s i on . The
f i s s i o n r e s u l t s i n t h e ^^^U a t o m s ' b r e a k
i n g u p i n t o n e w
f i s s i o n - p r o d u c t a t o m s of n e a r l y e q u al
m a s s , a nd s o m e
f r e e n e u t r o n s , a n d t h e r e l e a s e o f a l a r
g e a m o u n t o f e n e r g y .
A m o n g t h e e x p e c t e d f i s si o n p r o d u c t s t h
e r e a l w a y s a r e
s o m e x e n o n i s o t o p e s , a n d t h e a m o u n t of x
e n o n s o p r o d u c e d
i s s o m e t i m e s u s e d a s a m e a s u r e of t h e a m o
u n t of f i s s i o n
t h a t h a s t a k e n p l a c e . Yo u c a n i m a g i n e t h
e s u r p r i s e of t h e
s c i e n t i s t s wh e n n o x e n o n c o u l d b e f ou nd i
n t h e g a s e s f r o m
t h e m o l t e n - s a l t r e a c t o r e x p e r i m e n t s
, a l th o u g h o t h e r p r o d
u c t s s h o we d t h a t f i s s i o n h a d u n d o u b t e d
l y t a k e n p l a c e .
Puzzle Explained W i th the d i s c ov er y o f xeno n t e t r a
f lu o r id e
the puz z le w as ex p la ine d . It tu rn ed ou t tha t f r e e
f luo r ine i s
g e n e r a t e d in th e r e a c t o r - f u e l m i x t u r e
b y t he n e u t r o n i r r a
d i a t i o n . Un d e r c e r t a i n c o n d i t i o n s t h i
s f l u o r i n e c a n r e a c t
wi th the f i s s io n p r od uc t xenon to fo rm a xeno n f lu
o r i de . In
th os e c a se s w he re no xeno n was found , the con d i t ion
s had
b e e n r i g h t f o r x e n o n f l u o r i d e f o r m a t i
o n . T h i s w a s a n o t h e r
c a s e i n wh i c h a d i s c o v e r y i n o n e f ie l d of s
c i e n c e a n s w e r e d
a p r o b l e m i n a n o t h e r .
P e r h a p s t h e m o s t s t a r t l i n g e x p e r i m e n
t w i t h x e n o n a n d
f l u o r i n e w a s r e p o r t e d t o w a r d s t h e e n d
of 1 9 6 5 . Xe n o n a n d
f luo r ine when mix ed in a d ry g la s s f l a sk w i l l r e
a c t if the
m i x t u r e i s e x p o s e d to s u n l i g h t In t h i s c
a s e t h e e n e r g y p r o
v ided by the sun l igh t i s enough to p r od uc e the nee ded
f lu o
r i n e a t o m s . T h i s b e i n g t h e c a s e , o n e m a
y wo n d e r why i t
t oo k s o m a n y y e a r s to p r e p a r e t h e f i r s t n
o b le g a s c o m
p o u n d s . Se v e r a l e x p l a n a t i o n s h a v e b e e
n o f f e r e d , s u c h a s
the d if fi cu lty in ge t t ing tho rough ly d r i ed g la s s
w a re , and
lac k of know ledge of the t ec hn iqu es fo r hand l ing f luo
r ine
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Tab l e V
P H Y S IC A L P R O P E R T I E S O F T H E X E NO N F L U O R
I D E S
Compound
X e F j
XeF4
XeFe
Color of
Solid
C o l o r l e s s
C o l o r l e s s
C o l o r l e s s
Color of
Vapor
C o l o r l e s s
C o l o r l e s s
G r e e n i s h -
Yel low
Mel t ing
P o m t
(C)
129
1 1 7
49.5
Vapor
P r e s s u r e
at 25C
(mm)
4 .6
2 .5
27
Dens i ty
g m / c c
at 25C
4.32
4.04
3.41
an d r e ac t i v e f l u o r id e s . T h e s e u n d o u b ted
ly p l ay ed a p a r t ,
bu t the m aj or f ac to r wa s p roba b ly the l ack of an ade
qu a te
amount o f xenon . (Unt i l r ecen t ly xenon was no t genera l
ly
av a i l ab l e i n m o s t l ab o r a to r i e s b ecau s e o f
i t s h ig h co s t . )
T h e x e no n f l u o r i d e s a r e c o l o r l e s s c r y s
t a l l i n e m a t e r i a l s
a t r o o m t e m p e r a t u r e , b u t t he y r e a c t r e a
d i l y w i th m o i s t u r e .
F o r t h i s r e a s o n t h ey m u s t b e h an d led in t h o
r o u g h ly d r i e d
eq u ip m e n t an d a r e u s u a l l y m an ip u l a t ed in m
e ta l v acu u m
s y s t e m s . A ty p i c a l ex p e r im en ta l s e tu p i s
sh o w n o n p ag e 22 .
Th e n eces s i t y o f av o id in g a r eac t i o n w i th w a
te r ( h y d r o ly s i s )
i s e x t r e m e l y i m p o r t a n t , a s w e s h a l l s e
e l a t e r . P r o v i d i n g
th i s p r ec au t io n i s o b s e r v e d , t h e f l u o r id
e s a r e s t ab l e a t
r o o m t e m p e r a t u r e a nd c a n b e s t o r e d f o r p
r o l o n g e d p e r i o d s
in n i c k e l c o n t a i n e r s .
S o m e o f t h e p h y s i ca l p r o p e r t i e s o f t h e f
l u o r id e s a r e
g iven in Ta b le V . Eac h of the f l uo r id es wi l l r e a c
t w i th
h y d r o g e n , f o r m in g h y d r o g en f l u o r id e an
d l i b e r a t i n g e l e
m en ta l x en o n ; f o r ex am p le ,
XeF 4 + 2H2 Xe + 4H F
T h e r e l a t i v e e a s e of t h i s r e a c t i o n w i th
h y d r o g e n e s t a b l i s h e s
X eFg a s t h e m o s t r e ac t i v e of t h e x en o n f l u o
r id e s , an d X eF j
a s t h e l e a s t r e a c t i v e . T h i s o r d e r of r e a
c t i v i t y h a s b e e n
co n f i r m ed b y o th e r e x p e r im en t s , i n w h ich t
h e x en o n f l u o
r i d e s ac t a s f l u o r in a t i n g ag en t s . In ad d i
t i o n , it h a s b een
found t ha t both X eF j and XeF4 can be s to re d in thoro ugh
ly
d r i ed g l a s s co n t a i n e r s , b u t X eFg r e a c t s
ev en w i th d r y g l a s s
o r q u a r t z . N o te t h a t x en o n , in f o r m in g th e
t h r e e f l u o r id e s ,
exh ib i t s va lences o f 2 , 4 , and 6 .
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Oxygen Containing Compounds
U n d e r n o r m a l c o n d i t i o n s i t d o e s n ot a p p
e a r t o b e p o s
s ib l e t o o b t a in a c h e m ica l r e ac t i o n b e tw e
en o x y g en an d
xenon o r b e tw een oxygen and a xenon f lu or id e . In tho
se
c a s e s w h e r e o x yg e n h a s b e e n i n t r o d u c e d
i nt o a x e n o n -
co n ta in in g co m p o u n d th e i n t r o d u c t io n h a s
b ee n ach i ev ed
b y th e r e p l ac em en t of f l u o r in e . O ne of t h e f
i r s t o x y g en -
c o n t a i n i n g c o m p o u n d s t o b e d i s c o v e r e
d w a s x e n o n o x i d e
t e t r a f l u o r i d e , X e O F 4 . C h e m i s t s a t t e
m p t i n g t o s t o r e X e Fg
in g l a s s fo un d th a t a c l ea r , co lo r l e s s li q u
id w as f o r m ed
by re ac t io n of the XeFg wi th the g l a s s . The l iqu id w
as
ana lyz ed and found to have the fo rm ul a XeOF4. Th e
oxygen
h ad b e en o b t a in ed fr o m th e g l a s s , w h ich m a y
b e r e g a r d ed a s
s i l i co n d io x id e , S iOj . Th e r e ac t i v e f l u o r
in e in t h e X eFg
rep laced the oxygen in the
SiOj ,
conver t ing i t to S iF4 :
2XeF6 + SiOz = 2XeOF4 + SiF4
Sinc e f lu or in e h a s a va le nc e of 1 and oxygen a va le
nc e of 2 ,
2 f l u o r in e a t o m s h ad t o b e r em o v ed to a l l o w
th e i n s e r t i o n
of 1 oxygen a to m .
Th i s o x y g en - co n ta in in g co m p o u n d i s a l s o f
o r m ed w h en
X e Fg r e a c t s w i t h j u s t e n o u gh w a t e r t o p r
o v i d e f o r t h e r e
p l a ce m en t of 2 of t h e f l u o r in e a to m s . Th i s r
e ac t i o n m a y b e
w r i t t e n :
XeFg + HjO = XeOF4 + 2 H F
X e n on o x id e t e t r a f l u o r i d e i s s o m e w h a t
l e s s r e a c t i v e t h a n
X eF g , b u t i s m o r e r e ac t i v e t h an X eF 4 . I t m
a y b e k ep t u n
ch an g ed in d r i e d n i ck e l co n t a i n e r s , b u t i
t s l o w ly a t t a c k s
g l a s s o r q u a r t z .
The re ac t io n of XeFg wi th enough w a te r to p ro v id e fo
r
th e r e p l ac em en t of a l l 6 f l u o r in e a to m s w i
th o x y g en a t o m s
y i e l d s X eO s, x en o n t r i o x id e :
XeFg + SHjO = X eO j + 6 HF
X en on t r i o x id e a l s o r e s u l t s w h en X eO F 4 i s
a l l o w ed to r e
m a in i n co n t ac t w i th g l a s s f o r p r o lo n g ed p
e r io d s , o r w h en
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XeOF4 r e a c t s w i t h w a t e r . T h e r e a c t i o n of
Xe F4 wi th wa t e r
c a n a l s o r e s u l t i n t h e f o r m a t i o n o f
XeO^.
T h i s i s a s o m e
w h a t s u r p r i s i n g r e a c t i o n , h o w e v e r . I
n
XeO^
the xenon ha s
a va len ce of 6 , the xenon be in g com bine d wi th th re e
oxygen
a t o m s e a c h o f v a l e n c e 2 . W h e n
XeO^
i s fo rmed f rom XeFg
o r X eO Fi the va len ce of the xenon in the o r ig i na l c o
m
pou nds i s a l s o 6 . Ho we ver , when XeOs i s p re p a re d
f rom
XeF4, the va lenc e o f the xenon in the s t a r t in g m a te r
i a l i s
on ly 4 . Th i s type o f r eac t ion comes abou t by
disproportion-
ation
of the x enon a to m s; so m e of th em end up in a h i gh
er
va len ce s t a t e and so m e in a low er one , tha t i s , so
m e of
t h e x en o n a t o m s a r e o x i d i z e d a n d o t h e r s
a r e r e d u c e d . T h e
p r o d u c t i o n o f x e n o n t r i o x i d e f r o m x e n
o n t e t r a f l u o r i d e a n d
w a t e r m a y b e f o r m u l a t e d t h u s :
3XeF4 + 6H2O = Xe + 2Xe03 + 12HF
S ta r t in g off wi th 3 xenon a to m s eac h hav ing a va le
nc e of 4 ,
t h e p r o c e d u r e e n d s u p w i t h
1
xenon a tom of va len ce ze ro and
2 of va len ce 6 , thus ba la nc ing the v a l en ce s . In a lk
a l in e so lu
t i o n s , f o r e x a m p l e c a u s t i c s o d a , t h e d
i s p r o p o r t i o n a t i o n c a n
go a s t ep fu r th e r and y ie ld com pou nds con ta in ing
xenon
wi th a va lence o f 8 , such as sod ium perxena te , Na4Xe06
.
T h e p e r x e n a t e s a l t s r e a c t w i th c o n c e n t
r a t e d s u l p h u r i c a c id
to y ie ld the 8 -va len t xenon t e t ro x i de , Xe04 .
EX TR EM E CARE MUST BE TAK EN WITH BOTH O F THE
XE NON OXI DE S, B E C AUSE T HE Y AR E POW E R FUL E X
PLO SIVE S UNDER CER TA IN CONDITIONS. Xenon t r io x id e
i s r e l a t iv e l y s a fe in so lu t ion in w a t e r . When
the w a t e r ev ap
o ra te s , how eve r , the p u re xenon t r io x id e i s l ef
t in the fo rm
of c o l o r l e s s c r y s t a l s , wh i c h a r e a s p o we
r f u l a s T NT i n
th e i r ex p lo s ive p ow er Un l ike the c as e wi th TN T, i
t i s no t
known under wha t cond i t ions the c rys ta l s can be hand
led
s a f e l y , n o r e x a c t l y wh a t c a u s e s t h e m t o
e x p l o d e . T h i s
m a k e s wo r k i n g w i th x e n on t r i o x i d e
extremely hazardous.
M o r e o v e r , b e c a u s e t h e x e n on f l u o r i d e s
r e a c t w i t h m o i s t u r e
to g ive xenon t r io x i de , even w ork ing wi th these c o m
p o u n d s
c a n a l s o b e d a n g e r o u s . T h e m e t a l c o n t a
i n e r s ho wn i n F i g
u r e 7 w as da m ag ed by the exp los io n of about 100 m
g.
( 0 .0 0 3 5 oz . ) of x e n o n t r i o x i d e . E v e n e x p
e r i e n c e d an d c a r e
fu l s c ien t i s t s have been in ju red when work ing wi th
xenon
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Figure This nickel can, about 4 inches long and i % inches
iiide
photo IS approximately actual size), uas ruptured by detonation
of
100 mtlligianis of XeO j.
c o m p o u n d s . T h e s e , t h e n , a r e n o t m a t e r
i a l s to b e w o r k e d
wi th in a ba se m en t l ab or a t o r y in a ho m e, bu t shou
ld on ly
b e h a n d le d m w e l l - e q u i p p e d l a b o r a t o r i
e s by e x p e r i e n c e d
w o r k e r s w h o g i v e e v e r y r e g a r d to s a f e ty
p r e c a u t i o n s .
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More Complex Compounds
Me n t i o n h a s b e e n m a d e o f Xe P t Fg a n d s i m i l
a r c o m p o u n d s
i n wh i c h x e n o n c o m b i n e s w i t h m e t a l h e x a
f l u o r i d e s . T h e
e x a c t n a t u r e of t h e s e c o m p o u n d s i s h a r d
t o e l u c i d a t e a n d
i s s t i l l be ing inv es t iga ted . Bo th xenon d i f luo r
id e and xenon
hex af luo r ide wi l l r e a c t w i th a nu m be r of o t he r
f lu o r id es to
f o r m addition compounds. T ab le VI show s the fo rm ula s
of
s o m e o f t h e c o m p l e x e s t h a t h a v e b e e n r e
p o r t e d . Ap a r t
Table VI
COMPLEXES OF XENON AND KRYPTON FLUORIDES
N o b l e G a s
C o m p o u n d
C o m p l e x i n g
F l u o r i d e
N a F
K F
R b F
C s F
S b F j
A s F j
B F 3
T a F j
VF5
X e F 2
X e F 4
R a t i o o f N o b l e G a s C (
*
*
1 : 2
*
*
1 : 2
*
*
*
t
t
*
X e F g
3 m p o u n (
1 :2
1 :2
1 : 2
1 :1
1 :2
1 : 1
1 :2
1 : 1
2 : 1
1 :1
1 :1
t
2 : 1
X e O F 4 K r F g
i to C o m p l e x i n g F l u o r i d e
*
1 :3
1 : 6
2 : 3
1 : 3
2 : 3
1 : 1
1 : 2
t
t
2 .1
*
t
t
*
*
t
1 : 2
J
t
*No compou nd fo r m ed .
tHas no t been t r i ed .
tCompound fo rms ; fo rmu la no t ye t known .
U n s t a b le a b ov e - 2 0 C .
f r o m t h e i r c h e m i c a l c o m p o s i t i o n , a nd a
few p h y s i c a l p r o p
e r t i e s , n o t m u c h e l s e i s k no wn a b o u t t h e
s e c o m p l e x e s .
Xe n o n t e t r a f l u o r i d e d o e s n o t a p p e a r to
f o r m a s i m i l a r
s e r i e s o f a d d i t i o n c o m p o u n d s .
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COMPOUNDS OF OTHER NOBLE GASES
Radon
The ioniz ation po ten tial of ra don is the low est of any
of
the noble gases, which might lead one to think it would be
the mo st willing to form com pou nds. T his m ay in fact be
the case, but experiments wi th radon are severely ham
pe red be cau se of i ts high rad ioa ctiv ity. Work done
with
very sm all a m oun ts of m at er ia l (about one bil lionth
of
a gram ) ha s shown that radon ga s re a c ts with fluorine
at 400C to yield a compound that is not gaseous at room
temperature, as both radon and f luorine are. The course
of the reaction was followed only by monitoring with ra
diat ion detect ing inst ruments the movement of the radio
activity associated with one of the products of decay of the
radon. The formula of the compound produced has not been
de term ine d, and furth er investigation will be needed in
which la rg e r qu anti t ies of radon can be us ed . Th is wil
l
req uire elabo rate shielding to pro tec t the ex pe rim en
ters
from the high radioactivity.
Krypton
After xenon and radon, krjrpton should be the most likely
of the rem ainin g noble ga se s to form comp ound s. I ts io
niz a
tion potential is somewhat higher than that of either oxygen
or xenon, and it will not react with platinum, ruthenium,
or rhodium h exafluo rides (PtFg, RuFg, md RhFg, re s p e c
t ively).
The simple heating of krypton and fluorine also has
failed to prod uc e a com pound. Ho we ver, a krypton
fluoride
compound can be formed under the more drast ic experi
mental condit ions of passing an electric discharge or an
electron beam through a mixture of the 2 gases. The
krypton fluoride wil l decom pose alm os t as fast as i t is
form ed if it is left in the d isc h ar g e o r beam zo ne s.
But if
the con tainer is im m er se d in a cold bath the krypton
fluo
ride con den ses on the c on tainer w all , and is thus remo
ved
from the zone in which the energy is g en era ted . In thi s
way
krypton difluoride also ha s been produ ced, and pos sibly
krypton tetra flu orid e. The e vidence for the forma tion
of
the lat ter i s somewhat inconclusive, however .
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Krypton difluoride is a colorless, crystall ine compound
that decomposes into krypton and fluorine at room temper
ature. At the temperature of dry ice, 78C, krypton di
fluoride may be sto re d unchanged for prolonged per io ds
of time. Chemically, it is a much more reactive compound
than xenon difluoride, and in fact, it s fluorinating p ro p e
r
t ie s app ear to be even gr e at er than those of xenon he
xa
fluoride.
Helium
Neon and Argon
All evidence now available points to the fact that these
ga se s a re st i l l in er t . If one w ere to look at the pr
op er tie s
of the fluo rides of krypton we have ju st disc us se d, in co
m
pa ris on with those of the xenon flu ori de s, one would im
m ediately expect that fluor ide s of the th re e ligh test
noble
gases could be prepared only under extreme conditions,
and even then would be stable a t only low te m p er a tu re s
.
Attempts to prepare compounds have so far failed, but
who knows what may be found some day? Only a few years
ago the idea of a xenon fluoride seemed preposterous, too.
SHAPES OF MOLECULES
Solid State
In solid s, the m olecu les a re condensed to form cr y st al
s,
and the way in which the atom s a re ar ra ye d in the m ol
e
cu les may be determ ined by using be am s of X ray s or
ne utr on s. When such a beam is dir ect ed at a cr ys tal i
t
e i ther p as se s through the sp ace s between a toms und
is
turbed , or else i t str ik es an atom and is sc atte red or d
e
flected. The amount of sc at te rin g can be detected and m
ea
sured, giving a pattern that can be related to the location
of
the atoms and therefore to the structure of the crystal .
The de term ina tion of the actua l ar ra y of the ato m s
in
any unknown crystal has to be made in an indirect manner.
A gue ss is m ade of i ts prob able st ru ct u re and the pa
ttern
that this str uc tu re would prod uce i s calculated . This pa
t
te rn i s com pared with the exp erim enta l pa tter n. When
an
exact m atch is obtained, i t is app aren t the st ru ct ur e
is
known. T his used to be a long, tediou s op eratio n, but m
od
ern computer technology has simplified the process.
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T h e a t o m s of m a t e r i a l a r e s p r e a d o ut i n a
ll t h r e e d i
m e n s i o n s t h r o u g h o u t e v e r y c r y s t a l a n
d t h i s c o m p l e x i t y i n
t h e o r y co u ld le a d to v e r y c o m p l i c a t e d s t
r u c t u r e s . F o r
t u n a t e l y , i t t u r n s o u t t h a t t h e r e a r e c
e r t a i n a r r a y s of a t o m s
6
9
O
9
Xenon atoms
Fluorine atoms
-0
Figure 8 Crystal structure ofXeF^,
left, and XeF^.
t h a t r e p e a t t h e m s e l v e s th r o u g h o u t t h e
c r y s t a l l a t t i c e ; t h e s e
a r e c a l l e d u n i t c e l l s , a n d th e p r o b l e m i
s r e d u c e d t o o n e
of f ind ing the lo ca t io ns of the a to m s in eac h of the
un i t
c e l l s .
B o th X - r a y - d i f f r a c t i o n a n d n e u t r o n - d
i f f r a c t i o n t e c h n i q u e s
h a v e b e e n u s e d t o d e t e r m i n e t h e s t r u c t
u r e s of X e F2 a n d
XeF4 , and the X - r ay m e thod a lone ha s bee n use d fo r
XeOs .
F i g u r e 8 s h o w s t h e c r y s t a l s t r u c t u r e s
of X e F 2 a nd X e F 4 s o
de te r m in ed . The h igh r e ac t i v i t i e s o f XeFg ,
XeOF4, and
K r F 2 p r o d u c e p r o b l e m s w h en a n a t t e m p t i
s m a d e t o e x a m
i n e t h e i r s o l id p h a s e s t r u c t u r e s . S a m p
l e s t o b e e x a m i n e d
by X- ray t echn iques a r e u sua l ly loaded in to long , th
in g la ss
c a p i l l a r i e s . F i g u r e 9 s h o w s a s c i e n t i
s t p o s it i o n in g o n e
s u c h c a p i l l a r y i n a n X - r a y c a m e r a . A s X
e F g, X e O F 4 , a nd
K r F 2 a r e i n c o m p a t i b l e w i t h g l a s s , a n d
a l s o a r e m o s t e a s i l y
h a n d le d b e lo w r o o m t e m p e r a t u r e , t he y r e
q u i r e s p e c i a l
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Figure 9 Argon ne scientist Stanley Siegel positions a
capillary
containing XeF^ m an X-ray camera. The capillary is the
needle-
like object in the center of the picture.
t e c h n i q u e s , a nd t h e i r s o l i d - p h a s e s t r
u c t u r e s ha v e y e t t o b e
d e t e r m i n e d .
Gas Phase
W h e r e a s in t h e s o l id p h a s e t h e m o l e c u l e
s f o r m i n g t h e
c r y s t a l a r e q u i t e c lo s e t o g e th e r an d can i
n f lu en ce o n e
an o t he r , in the g as ph as e they a r
