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