Fondamenti di Chimica & Educazione Ambientale Lecture 2

Atoms, Molecules & Ions

“Leucippo è l’iniziatore di quel famigerato sistema atomistico, che, risuscitato modernamente, è passato come principio fondamentale dell’indagine razionale sulla natura. Esaminato questo sistema per sè, lo troviamo bensì assai povero e tale da offrire poco alla ricerca; ma deve essere riconosciuto a Leucippo il grande merito di aver distinto, come si dice comunemente nella nostra fisica, le qualità generali dei corpi sensibili, o le primarie dalle secondarie, o le essenziali dalle non essenziali.”

Georg Wilhelm Friedrich Hegel “Lezioni di Storia della Filosofia” La Nuova Italia Editrice Vol. 1, p. 231, 1963

Georg Wilhelm Friedrich Hegel Jena 1805 - 1806

Elements

These are found and listed on the Periodic Table

Pure substances composed of only one type of atom.

Atoms

The smallest particle of an element that retains all of the properties of the element.

u A species of atoms; all atoms with the same number of protons in the atomic nucleus.

u A pure chemical substance composed of atoms

with the same number of protons in the atomic nucleus. Sometimes this concept is called the elementary substance as distinct from the chemical element as defined under 1, but mostly the term chemical element is used for both concepts

Chemical Element

Molecules

A sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by chemical bonds.

Compounds

A pure substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass.

08/02/1834 - 20/01/1907 Dmitrij Ivanovič Mendeleev

Ti=50 Zr=90 ?[2]=180 V=51 Nb=94 Ta=182 Cr=52 Mo=96 W=186 Mn=55 Rh=104,4[3] Pt=197,4[4] Fe=56 Ru=104,4 Ir=198 Ni=Co=59 Pd=106,6 Os=199

H=1[5] Cu=63,4 Ag=108 Hg=200 Be=9,4 Mg=24 Zn=65,2 Cd=112 B=11 Al=27,4 ?[6]=68 Ur=116[7] Au=197? C=12 Si=28 ?[8]=70 Sn=118 N=14 P=31 As=75 Sb=122 Bi=210? O=16 S=32 Se=79,4 Te=128? F=19 Cl=35,5 Br=80 J=127[9]

Li=7 Na=23 K=39 Rb=85,4 Cs=133 Tl=204 Ca=40 Sr=87,6 Ba=137 Pb=207 ?[10]=45 Ce=92[11] ?Er=56 La=94 ?Yt=60 Di=95 ?In=75,6 Th=118?

Pnictogens

The Periodic table A

lkal

i M

etal

s Alkaline Earths

Transition Metals

Halogens

Nob

le G

ases

Lanthanides and Actinides

Main Group

Main Group Calchogens

http://www.meta-synthesis.com/webbook/35_pt/pt_database.php?Button=post-2000+Formulations

A given compound always contains the same proportion of elements, by mass.

Dalton, John 1766-1844 Lithograph

Dalton’s Atomic Theory 1.  Each element is made up of atoms;

2.  Atoms of a given element are identical while atoms of different elements differ;

3.  Chemical compounds are made up of specific whole number ratios of atoms;

4.  Reactions involve reorganization of atoms. Atoms do not change.

Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies, and the Proportions in Which They Enter Into These Compound Lorenzo Romano Amedeo Carlo Avogadro Journal de Physique, 1811, 73, 58-76

Atomic Masses:

A relative comparison of the average masses of atoms.

Avogadro’s Hypothesis (1811):

Equal volumes of a gas contain the same number of atoms under the same conditions of T and P.

Atomic Structure

Until the end of the 19th century, not much was known about the atom.

BUT, WHAT WAS THE ATOM?

People knew: i)  There were different elements ii) Each had a unique mass and reacted in

a special way.

Introduction to

Atomic Theory

The atom was a black box. People knew there were different substances (different boxes), each with unique properties, but “why” was a question.

Sir Joseph John Thomson (1856 - 1940) was awarded the 1906 Nobel Prize. “… in recognition of the great merits of his theoretical and experimental investigations on

the conduction of electricity by gases.”

The early experiments of J.J. Thompson cast light on the question. An evacuated tube, containing a small amount of a gas was attached to a power supply

LVIII. On the Masses of the Ions in Gases at Low Pressures. By J.J. Thomson, M.A., F.R.S.,

Cavendish Professor of Experimental Physics, Cambridge Philosophical Magazine

December 1899 Series 5, Vol. 48, No. 295, p. 547-567

+ -

The same cathode ray was seen no matter what the tube was filled with.

Conclusion: The cathode ray was a fundamental

unit of all materials.

+ -

Through a number of experiments, J.J. was able to determine the charge (e) to mass (m)

ratio of the cathode ray.

e/m = some number e/m =-1.75881962 x 1011 C/Kg.

+ -

By similar analysis he was also able to look at the canal Ray, the positive piece “left over” after the Cathode ray had been ejected.

e/m was different for each gas that was in the evacuated tube.

+

+

Starting with two different atomic “Boxes”

Same

Positive pieces “left over” are different

qm

= ν02Δy

EdL = EΔyB2dL

x = ν0t

tanα = dy

dx⎛⎝⎜

⎞⎠⎟x=d

= qm

⎛⎝⎜

⎞⎠⎟

Edν0

2 = ΔyL

qm

= ν02Δy

EdL

y = 1

2qm

⎛⎝⎜

⎞⎠⎟ Et 2 = 1

2qm

⎛⎝⎜

⎞⎠⎟ E x

ν0

⎛⎝⎜

⎞⎠⎟

2

ν = E

B Eq = qνB

qm

= ν02Δy

EdL = EΔyB2dL

qm

= ν02Δy

EdL

At this point J.J was ready to talk about the constitution of the atom: a large “sea” of positive charge, imbedded with electrons.

The Plum Pudding Model

The e/me ratio (-1.75881962 x 1011 C/Kg)

What does it tell us?

Millikan’s Oil Drop Experiment

Small droplets of oil, each given a negative charge, are pushed upward by an electric force while they simultaneously fall downward under the influence of gravity. Controlled variations of the electric field show that the negative charge on each drop is always a multiple of a certain value e.

Zap with radiation

Experimental data indicate that: e = -1.60217733 x 10-19 C; me = 9.1093897 x 10-31 kg.

Millikan’s Oil Drop Experiment

Back to Plum Pudding

A “sea of positive charge” containing embedded electrons

They shot a particles through the foil. This should be equivalent to shooting a howitzer shell through paper tissue.

The gold foil experiment:

United Kingdom and New Zealand Victoria University Manchester, U. K. b. 1871 (in Nelson, New Zealand) d. 1937 Nobel prize in Chemistry 1908

They shot a particles through the foil. This should be equivalent to shooting a howitzer shell through paper tissue.

The gold foil experiment:

A solar system model of the atom.

Electrons circle the nucleus.

The Concept of Atomic Number Z = number of protons

You will notice that most of the Periodic Table is well-arranged according to Atomic Mass.

X Z

Z = atomic number = # protons

A = atomic mass = #protons + neutrons

A

Different atomic masses

Isotopes: Atoms with the same number of protons (Z) but different number of neutrons (A-Z) X

A

Protium mass: 1.007825 u Deuterium mass: 2.01410178 u

Composition of the Atom

But, the nucleus is made of protons and neutrons

Electrons

Nucleus

Li

2e- 1e-

Be

2e- 2e- ......

Ne

2e- 8e-

2e-

Na

2e- 8e-

Mg

2e- 8e- ......

Ar

2e- 8e-

1e- 8e-

K

2e- 8e-

8e-

Ca

2e- 8e-

8e-

1e-

......

2e-

Law of Definite Proportion A given compound always contains the same proportion of elements, by mass.

Law of Multiple Proportions When two elements form two different

compounds, the mass ratio of the elements in one compound is related to the the mass ratio of the elements in the latter compound by a small whole number.

Mass of Oxygen combining with 1 gram of Nitrogen

Compound A 1.1428 g Compound B 2.2857 g

Divide by smallest number to get ratios:

1.1428/1.1428 = 1 2.2857/1.1428 = 2

NO N2O2

We really do not know the real formula

NO2 N2O4

Lecture 2 End

Lecture 2 End