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
add 12
O C O
O C O
more e¯
4 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
F
B
F
add 18
F
more e¯
F
F
B
F
6 e¯
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)
F
F
B
F
F
F
B
F
3. CH4
(a) carbon tetrahydride methane!
(b) CH4 has 4+1+1+1+1 (=8) valence electrons
H
H C H
H
8 e¯
done !
H
H C H
H
(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)
H
H
H
H
C
H
H
H
C
H
4. PCl5
(a) phosphorus pentafluoride
(b) has 5+7+7+7+7+7 (=40) valence electrons
Cl
Cl
P
Cl
Cl
Cl
add 30
Cl
more e¯
Cl
Cl
P
Cl
Cl
10 e¯
(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)
Cl
Cl
Cl
Cl
P
Cl
Cl
Cl
P
Cl
Cl
Cl
5. SF6
(a) sulfur hexafluoride
(b) has 6+7+7+7+7+7+7 (=48) valence electrons
F
F
F
S
F
add 36
F
F
more e¯
F
F
F
S
F
F
F
12 e¯
(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)
F
F
F
F
S
F
6. CCl4
F
F
F
F
S
F
F
F
(a) carbon tetrachloride
(b) has 4+7+7+7+7 (=32) valence electrons
Cl
Cl C Cl
Cl
Cl
Cl C Cl
Cl
add 24
more e¯
8 e¯
(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)
Cl
Cl
Cl
C
Cl
Cl
Cl
Cl
C
7. CH2Cl2
(a) dichloromethane
(b) has 4+1+1+7+7 (=20) valence electrons
H
add 12
H C Cl
more e¯
Cl
Cl
H
H C Cl
Cl
8 e¯
(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)
Cl
H
H
C
Cl
Cl
H
H
C
Cl
8. IF
(a) iodine fluoride
(b) has 7+7 (=14) valence electrons
add 12
I
F
2 e¯
I
more e¯
(c) has only two atoms, so must be linear
(d) linear (doesn’t really have a central atom)
(e)
I
F
9. CS2
(a) carbon disulfide
(b) has 4+6+6 (16) valence electrons
add 12
S C S
more e¯
4 e¯
I
F
F
S C S
(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
F
F
P
F
F
add 36
F
F
more e¯
F
F
F
F
P
F
F
12 e¯
(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)
F
F
F
F
P
F
F
F
F
F
F
P
F
F
11. AlCl3
(a) aluminum trichloride
(b) has 3+7+7+7 (=24) valence electrons)
Cl
Cl
Al
add 18
Cl
more e¯
Cl
Cl
Al
Cl
6 e¯
(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)
Cl
Cl
Al
Cl
Cl
Cl
Al
Cl
12. AlCl4¯
(a) aluminum tetrachloride
(b) has 3+7+7+7+7+1
Cl
Cl Al Cl
Cl
8 e¯
add 24
more e¯
Cl
Cl Al Cl
Cl
(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)
Cl
Cl
Cl
Al
Cl
Cl
Cl
Cl
Al
Cl
13. SiCl4
(a) silicon tetrachloride
(b) has 4+7+7+7+7 (=32) valence electrons
Cl
Cl Si Cl
Cl
Cl
Cl Si Cl
Cl
add 24
more e¯
8 e¯
(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)
Cl
Cl
Cl
Si
Cl
Cl
Cl
Cl
Si
Cl
14. SO3
(a) sulfur trioxide
(b) has 6+6+6+6 (=24) valence electrons
O
O
S
O
add 18
O
more e¯
O
6 e¯
S3
O
O
O
S
O
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)
O
O
S
15. H2O
(a) dihydrogen monoxide water
(b) has 6+1+1 (=8 valence) electrons
O
O
O
S
O
H
O H
H
O H
add 4
more e¯
4 e¯
(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)
H
H
O
O
H
16. NH3
(a) nitrogen trihydride ammonia
(b) has 5+1+1+1 (=8) valence electrons
H
H N H
H
H
H N H
add 2
more e¯
6 e¯
(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)
H
H
N
H
H
H
N
H
H
H
N
H
trigonal
pyramidal
tetrahedral
17. NH4+
(a) ammonium
(b) has 5+1+1+1+1–1 (=8) valence electrons
H
H N H
done !
H
8 e¯
(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)
H
H
H
18. NO2¯
N
H
H
H
H
N
H
(a) nitrite
(b) has 5+6+6+1 (=18) valence electrons
O
N
add 14
O
O
N
more e¯
O
N
O
O
4 e¯
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 nonoctet. 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)
O
N
O
N
O
O
bent
trigonal
planar
19. O3
(a) ozone
(b) has 6+6+6 (=18) valence electrons
O
O
2
add 14
O
O
more e¯
O
O
O
O
O
4 e¯
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)
O
O
O
trigonal
planar
20. SnCl4
(a) tin tetrachloride
(b) has 4+7+7+7+7 (=32) valence electrons
O
O
O
bent
Cl
Cl Sn Cl
Cl
Cl
Cl Sn Cl
Cl
add 24
more e¯
8 e¯
(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)
Cl
Cl
Cl
Cl
Sn
Cl
Cl
Cl
Sn
Cl
21. NO3¯
(a) nitrate anion
(b) has 5+6+6+6+1 (=24) valence electrons
O
O
N
O
add 18
O
more e¯
O
6 e¯
N2
O
N
O
O
O
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)
O
O
N
O
O
O
N
O
22. SF4
(a) sulfur tetrafluoride
(b) has 6+7+7+7+7 (=34) valence electrons
F
F
F
S
F
add 26
F
more e¯
F
F
S
F
8 e¯
(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)
F
F
F
F
S
F
S
F
F
F
trigonal bipyramidal
"see-saw"
23. ClF3
(a) chlorine trifluoride
(b) has 7+7+7+7 (=28) valence electrons
F
Cl
add 22
F
more e¯
F
6 e¯
F
Cl
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)
F
F
Cl
F
Cl
F
F
F
trigonal bipyramidal
"T-shaped"
24. XeF2
(a) xenon diflouride
(b) has 8+7+7 (=22) valence electrons
F
Xe
add 18
F
more e¯
4 e¯
F
Xe
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)
F
F
Xe
Xe
F
F
trigonal bipyramidal
25. BrF5
linear
(a) bromine pentafluoride
(b) has 7+7+7+7+7+7 (=42) valence electrons
F
F
F
Br
add 32
F
F
more e¯
F
F
F
Br
F
F
10 e¯
(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)
F
F
F
F
Br
F
F
F
F
F
F
Br
square
pyramidal
octahedral
26. XeF4
(a) xenon tetrafluoride
(b) has 8+7+7+7+7 (=36) valence electrons
F
F
Xe
add 32
F
F
more e¯
F
F
Xe
F
F
8 e¯
(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)
F
F
Xe
F
F
F
F
Xe
F
F
square
planar
octahedral
27. IF5
(a) iodine pentafluoride
(b) has 7+7+7+7+7+7 (=42) valence electrons
F
F
F
I
10 e¯
F
F
add 32
more e¯
F
F
F
I
F
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)
F
F
F
F
F
F
F
F
I
F
F
I
square
pyramidal
octahedral
28. GaH3
(a) gallium trihydride
(b) has 3+1+1+1 (=6) valence electrons
H
H
Ga
H
done !
6 e¯
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)
H
H
Ga
H
H
H
Ga
H
29. ICl2¯
(a) iodine dichloride
(b) has 7+7+7+1 (=22) valence electrons
Cl
I
Cl
more e¯
4 e¯
Cl
add 18
I
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)
Cl
Cl
I
I
Cl
Cl
trigonal bipyramidal
linear
30. BrF4¯
(a) bromine tetrafluoride
(b) has 7+7+7+7+7+1 (=36) valence electrons
F
Br
F
add 28
F
more e¯
F
F
F
Br
F
F
8 e¯
(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)
F
F
Br
F
F
F
F
Br
square
planar
octahedral
F
F