Valence Shell Electron Pair Repulsion Model & the Shape of Molecules

For each of the following provide (a) the chemical name, (b) the Lewis structure, (c) the name of the “electron pair” geometry, (d) the name for the “molecular” geometry, (e) a 3D drawing of the compound. 1. CO2

(a) carbon dioxide (b) CO2 has 4+6+6 (=16) valence electrons

O C O O C O

4 e¯

add 12

more e¯

(c) C is attached to two atoms and has no lone pairs; therefore the electron pair geometry is

linear (d) because C has no lone pairs the molecular geometry is also linear (e)

O C O O C O

2. BF3 (a) boron trifluoride (b) BF3 has 3+7+7+7 (=24) valence electrons

BF

FF

6 e¯

add 18

more e¯BF

FF

Note that boron as shown in this Lewis structure violates the octet. Other resonance forms could be written that give boron a complete octet by sharing the electrons in the fluorine lone pair.

(c) B is attached to three atoms and has no lone pairs; therefore the electron pair geometry is trigonal planar

(d) because B has no lone pairs the molecular geometry is also trigonal planar (e)

BF

FF BF

FF

3. CH4 (a) carbon tetrahydride methane! (b) CH4 has 4+1+1+1+1 (=8) valence electrons

CH

HHH

8 e¯

done ! CH

HHH

(c) C is attached to four atoms and has no lone pairs; therefore the electron pair geometry is tetrahedral

(d) because C has no lone pairs the molecular geometry is also tetrahedral (e)

C

H

HH

HC

H

HH

H

4. PCl5

(a) phosphorus pentafluoride (b) has 5+7+7+7+7+7 (=40) valence electrons

P

Cl

Cl

Cl

10 e¯

add 30

more e¯Cl

Cl

P

Cl

Cl

ClCl

Cl

(c) P is attached to five atoms and has no lone pairs; therefore the electron pair geometry is

trigonal bipyramidal (d) because P has no lone pairs the molecular geometry is also trigonal pyramidal (e)

P Cl

Cl

Cl

Cl

ClP Cl

Cl

Cl

Cl

Cl

5. SF6

(a) sulfur hexafluoride (b) has 6+7+7+7+7+7+7 (=48) valence electrons

S

F

FF

FF

F

12 e¯

add 36

more e¯S

F

FF

FF

F

(c) S is attached to six atoms and has no lone pairs; therefore the electron pair geometry is

octahedral (d) because S has no lone pairs the molecular geometry is also octahedral (e)

S

F

F

FFF

FS

F

F

FFF

F

6. CCl4

(a) carbon tetrachloride (b) has 4+7+7+7+7 (=32) valence electrons

CCl

ClClCl

8 e¯

add 24

more e¯CCl

ClClCl

(c) C is attached to four atoms and has no lone pairs; therefore the electron pair geometry is

tetrahedral (d) because C has no lone pairs the molecular geometry is also tetrahedral (e)

C

Cl

ClCl

ClC

Cl

ClCl

Cl

7. CH2Cl2

(a) dichloromethane (b) has 4+1+1+7+7 (=20) valence electrons

CH

ClClH

8 e¯

add 12

more e¯CH

ClClH

(c) C is attached to four atoms and has no lone pairs; therefore the electron pair geometry is

tetrahedral (d) because C has no lone pairs the molecular geometry is also tetrahedral (e)

C

Cl

ClH

HC

Cl

ClH

H

8. IF

(a) iodine fluoride (b) has 7+7 (=14) valence electrons

I F

2 e¯

add 12

more e¯I F

(c) has only two atoms, so must be linear (d) linear (doesn’t really have a central atom) (e)

I F I F 9. CS2

(a) carbon disulfide (b) has 4+6+6 (16) valence electrons

S C S S C S

4 e¯

add 12

more e¯

(c) C is attached to two atoms and has no lone pairs; therefore the electron pair geometry is linear

(d) because C has no lone pairs the molecular geometry is also linear (e)

S C S S C S

10. PF6¯ (a) phosphorus hexafluoride (b) has 5+7+7+7+7+7+7+1 (=48) valence electrons

P

F

FF

FF

F

12 e¯

add 36

more e¯P

F

FF

FF

F

(c) P is attached to six atoms and has no lone pairs; therefore the electron pair geometry is

octahedral (d) because P has no lone pairs the molecular geometry is also octahedral (e)

P

F

F

FFF

FP

F

F

FFF

F

11. AlCl3

(a) aluminum trichloride (b) has 3+7+7+7 (=24) valence electrons)

AlCl

ClCl

6 e¯

add 18

more e¯AlCl

ClCl

(c) Al is attached to three atoms and has no lone pairs; therefore the electron pair geometry is

trigonal planar (d) because Al has no lone pairs the molecular geometry is also trigonal planar (e)

AlCl

ClCl AlCl

ClCl

12. AlCl4¯ (a) aluminum tetrachloride (b) has 3+7+7+7+7+1

AlCl

ClClCl

8 e¯

add 24

more e¯AlCl

ClClCl

(c) Al is attached to four atoms and has no lone pairs; therefore the electron pair geometry is tetrahedral

(d) because Al has no lone pairs the molecular geometry is also tetrahedral (e)

Al

Cl

ClCl

ClAl

Cl

ClCl

Cl

13. SiCl4

(a) silicon tetrachloride (b) has 4+7+7+7+7 (=32) valence electrons

SiCl

ClClCl

8 e¯

add 24

more e¯SiCl

ClClCl

(c) Si is attached to four atoms and has no lone pairs; therefore the electron pair geometry is

tetrahedral (d) because Si has no lone pairs the molecular geometry is also tetrahedral (e)

Si

Cl

ClCl

ClSi

Cl

ClCl

Cl

14. SO3

(a) sulfur trioxide (b) has 6+6+6+6 (=24) valence electrons

SO

OO

6 e¯

add 18

more e¯S

O

OOSO

OO3

Note that we make sure to first give oxygen its octet because it is more electronegative. However, the formal charge on each monovalent oxygen (6-1-6 = –1). Since sulfur has no lone pairs its formal charge is 6–3–0 = 3+. Another resonance form results when the oxygen shares its lone pairs with sulfur to give pi bonds and no formal charge. Other resonance structures could be written.

(c) S is attached to three atoms and has no lone pairs; therefore the electron pair geometry is trigonal planar

(d) because S has no lone pairs the molecular geometry is also trigonal planar (e)

SO

OO SO

OO

15. H2O (a) dihydrogen monoxide water (b) has 6+1+1 (=8 valence) electrons

OH

H

4 e¯

add 4

more e¯OH

H

(c) O is attached to two atoms and has two lone pairs; therefore the electron pair geometry is

tetrahedral (d) because O has two lone pairs the molecular geometry is bent (e)

O

H

HO

H

H

16. NH3

(a) nitrogen trihydride ammonia (b) has 5+1+1+1 (=8) valence electrons

NH

H

6 e¯

add 2

more e¯H N

HHH

(c) N is attached to three atoms and has one lone pair; therefore the electron pair geometry is

tetrahedral (d) because N has one lone pair the molecular geometry is trigonal pyramidal (e)

NH

HH

NH

HH

NH

HH

tetrahedral trigonalpyramidal

17. NH4+ (a) ammonium (b) has 5+1+1+1+1–1 (=8) valence electrons

N

H

H

8 e¯

H

Hdone !

(c) N is attached to four atoms and has no lone pairs; therefore the electron pair geometry is

tetrahedral (d) because N has no lone pairs the molecular geometry is also tetrahedral (e)

NH

HH

H

NH

HH

H

18. NO2¯

(a) nitrite (b) has 5+6+6+1 (=18) valence electrons

NO

O

4 e¯

add 14

more e¯NO

ONO

O

Note that if N has two bonds and a lone pair, it has a formal charge of 1+ (5–2–2 ), while monovalent oxygen has a formal charge of 1– (6–1–6). The first resonance structure is non-octet. Sharing one of the oxygen lone pairs with nitrogen allows it to satisfy the octet rule as shown in the second resonance structure.

(c) N is attached to two atoms and has one lone pair; therefore the electron pair geometry is trigonal planar

(d) because N has one lone pair the molecular geometry is bent (e)

NO

O NO

O

trigonalplanar

bent

19. O3

(a) ozone (b) has 6+6+6 (=18) valence electrons

OO

4 e¯

add 14

more e¯O O

OOO

OO

2

Note that if the central O has two bonds and a lone pair, it has a formal charge of 2+ (6–2–2), while monovalent oxygen has a formal charge of 1– (6–1–6). This leaves the oxygen of the first resonance structure without its non-octet. Sharing one of the terminal oxygen lone pairs with nitrogen allows it to satisfy the octet rule as shown in the second resonance structure.

(c) the central O is attached to two atoms and has one lone pair; therefore the electron pair geometry is trigonal planar

(d) because O has one lone pair the molecular geometry is bent (e)

trigonalplanar

bent

OOO

OOO

20. SnCl4

(a) tin tetrachloride (b) has 4+7+7+7+7 (=32) valence electrons

SnCl

ClClCl

8 e¯

add 24

more e¯SnCl

ClClCl

(c) Sn is attached to four atoms and no lone pairs; therefore the electron pair geometry is

tetrahedral (d) because Sn has no lone pairs the molecular geometry is tetrahedral (e)

Sn

Cl

ClCl

ClSn

Cl

ClCl

Cl

21. NO3¯

(a) nitrate anion (b) has 5+6+6+6+1 (=24) valence electrons

NO

OO

6 e¯

add 18

more e¯N

O

OONO

OO2

Note that if N has three bonds and no lone pair (first resonance structure), it has a formal charge of 2+ (5–3), while monovalent oxygen has a formal charge of 1– (6–1–6). The first resonance structure is non-octet. Sharing one of the oxygen lone pairs with nitrogen allows it to satisfy the octet rule as shown in the second resonance structure.

(c) N is attached to three atoms and no lone pairs; therefore the electron pair geometry is trigonal planar

(d) because N has no lone pairs the molecular geometry is trigonal planar (e)

NO

OO NO

OO

22. SF4 (a) sulfur tetrafluoride (b) has 6+7+7+7+7 (=34) valence electrons

S

F

F

8 e¯

add 26

more e¯F

F

S

F

FF

F

(c) S is attached to four atoms and one lone pair; therefore the electron pair geometry is

trigonal bipyramidal (d) because S has one lone pair the molecular geometry is see-saw (e)

S

F

F

F

F S

F

F

F

F

trigonal bipyramidal "see-saw" 23. ClF3

(a) chlorine trifluoride (b) has 7+7+7+7 (=28) valence electrons

Cl

F

F

6 e¯

add 22

more e¯Cl

F

F

F

F

(c) Cl is attached to three atoms and two lone pairs; therefore the electron pair geometry is

trigonal bipyramidal (d) because Cl has two lone pairs the molecular geometry is T-shaped (e)

Cl F

F

F

Cl

F

F

trigonal bipyramidal "T-shaped"

F

24. XeF2

(a) xenon diflouride (b) has 8+7+7 (=22) valence electrons

XeF

4 e¯

add 18

more e¯Xe

F

F

F

(c) Xe is attached to two atoms and three lone pairs; therefore the electron pair geometry is

trigonal bipyramidal (d) because Xe has three lone pairs the molecular geometry is linear (e)

Xe

F

F

Xe

F

F

trigonal bipyramidal linear 25. BrF5

(a) bromine pentafluoride (b) has 7+7+7+7+7+7 (=42) valence electrons

Br

F

F

FF

F

10 e¯

add 32

more e¯Br

F

F

FF

F

(c) Br is attached to five atoms and one lone pair; therefore the electron pair geometry is

octahedral (d) because Br has one lone pair the molecular geometry is square pyramidal (e)

Br

F

FFF

FBr

F

FFF

F

octahedral

squarepyramidal

26. XeF4

(a) xenon tetrafluoride (b) has 8+7+7+7+7 (=36) valence electrons

XeF

FF

F

8 e¯

add 32

more e¯Xe

F

FF

F

(c) Xe is attached to four atoms and two lone pairs; therefore the electron pair geometry is

octahedral (d) because Xe has two lone pairs the molecular geometry is square planar (e)

XeFFF

FXe

FFF

F

octahedral

squareplanar

27. IF5

(a) iodine pentafluoride (b) has 7+7+7+7+7+7 (=42) valence electrons

I

F

F

FF

F

10 e¯

add 32

more e¯I

F

F

FF

F

(c) I is attached to five atoms and one lone pair; therefore the electron pair geometry is octahedral

(d) because I has one lone pair the molecular geometry is square planar (e)

I

F

FFF

FI

F

FFF

F

octahedral

squarepyramidal

28. GaH3

(a) gallium trihydride (b) has 3+1+1+1 (=6) valence electrons

GaH

HH

6 e¯

done !

Note that this is a violation of the octet. Gallium is in the same group as boron and aluminum, which also form trivalent non-octet structures (see problems #2 and #11).

(c) Ga is attached to three atoms and has no lone pairs; therefore the electron pair geometry is trigonal planar

(d) because Ga has no lone pairs the molecular geometry is trigonal planar (e)

GaH

HH GaH

HH

29. ICl2¯ (a) iodine dichloride (b) has 7+7+7+1 (=22) valence electrons

ICl

4 e¯

add 18

more e¯I

Cl

Cl

Cl

aluminum, which also form trivalent non-octet structures.

(c) I is attached to two atoms and has three lone pairs; therefore the electron pair geometry is trigonal planar

(d) because I has three lone pairs the molecular geometry is linear (e)

I

Cl

Cl

I

Cl

Cl

trigonal bipyramidal linear 30. BrF4¯

(a) bromine tetrafluoride (b) has 7+7+7+7+7+1 (=36) valence electrons

BrF

FF

F

8 e¯

add 28

more e¯Br

F

FF

F

(c) Br is attached to four atoms and has two lone pairs; therefore the electron pair geometry is

octaehedral (d) because Br has two lone pairs the molecular geometry is square planar (e)

BrFFF

FBr

FFF

F

octahedral

squareplanar