162 ❯ STEP 4. Review the Knowledge You Need to Score High
- The above diagram illustrates a portion of a mol-
ecule of buckminsterfullerene (a “Bucky ball”).
The corners are carbon atoms. Each carbon atom
has four covalent bonds, allowing it to follow the
octet rule. The structure also shows many single
and double bonds. Carbon-carbon double bonds
are shorter than carbon–carbon single bonds. In
an analysis of the structure of buckminsterfuller-
ene, all carbon–carbon bonds are found to be
the same length and not a combination of short
double bonds and longer single bonds. Which of
the following best explains this observation?
(A) VSEPR
(B) resonance
(C) hybridization
(D) experimental error
20.
ONO−
O NO ONO+There are three nitrogen–oxygen species known
with a 1:2 nitrogen-to-oxygen ratio. The Lewis
electron-dot diagrams for these three nitrogen
oxygen species are shown in the above diagram.
Which of the three has the largest bond angle?
(A) NO 2 -
(B) NO 2
(C) NO 2 +
(D) All have a 180° angle.❯ Answers and Explanations
- B—The Lewis (electron-dot) structure has five
bonding pairs around the central Sb and no lone
pairs. VSEPR predicts this number of pairs to
give a trigonal bipyramidal structure. - B—All bonds except those in CO are single
bonds. The CO bond is a triple bond. Triple
bonds are shorter than double bonds, which are
shorter than single bonds. Drawing Lewis struc-
tures might help you answer this question. - A—Answers B through D contain molecules or
ions with double or triple bonds. Double and
triple bonds contain p bonds. Water has only
single (s) bonds. If any of these are not obvious
to you, draw a Lewis structure. - C—Use VSEPR; only the tetrahedral SiF 4 is
nonpolar. The other materials form square
pyramidal (IF 5 ), T–shaped (IF 3 ), and irregular
tetrahedral (SeF 4 ) shapes and are therefore polar.
5. C—All other answers involve species containing
only single bonds. Substances without double or
triple bonds seldom need resonance structures.
6. C—Resonance causes bonds to have the same
average length.
7. D—Many organic molecules are nonpolar.
Nonpolar substances are held together by weak
van der Waals attractions.
8. D—Lewis structures are required. You may
not need to draw all of them. A and B have
one unshared pair, whereas C does not have an
unshared pair. D has two unshared pairs of elec-
trons.
9. B—For an atom to exceed an octet of electrons,
it must be in the third period or lower on the
periodic table. The elements N, C, and O are in
the second period; therefore, the answers con-
taining these elements contain an element that