164 ❯ STEP 4. Review the Knowledge You Need to Score High
❯ Answer and Explanation
(a) Silicon tetrafluoride is the only one of the three compounds that is not polar.
Si
SiF 4 : four bonding
pairs and no
lone pairs
Tetrahedral
F F
F
F
Xe
XeF 4 : four bonding
pairs and two
lone pairs.
Square planar
F F
F
F
S
SF 4 : four bonding
pairs and one
lone pair.
Seesaw
F F
F
F
You get 1 point if you correctly predict only SiF 4 to be nonpolar. You get 1 additional
point for each correct Lewis structure.
(b) The order is KrF 2 < SeF 2 < SnF 2.
F Kr F
FSe
F Sn 2
2+ −
F
KrF 2 : two bonding
pairs and three
nonbonding pairs
SeF 2 : two bonding
pairs and two
nonbonding pairs
SnF 2 : ionic
The Lewis structure indicates that KrF 2 is nonpolar. Thus, it has only very weak
London dispersion forces between the molecules. SeF 2 is polar, and the molecules are
attracted by dipole–dipole attractions, which are stronger than London. SnF 2 has the
highest melting point because of the presence of strong ionic bonds.
You get 1 point for the order and 1 point for the discussion.
(c) It is possible to draw the following resonance structures for the oxalate ion. The pres-
ence of resonance equalizes the bonds.
C
O
O^2 −
C
O
O
C
O
O^2 −
C
O
O O
O
C
O O
2 −
C
O
O
C
2 −
C
O
O
You get 1 point for any correct Lewis structure for C 2 O 42 - and 1 point for showing
or discussing resonance.