12 . C
For a substance to participate in hydrogen bonding, it must have hydrogen atoms attached to
F, O, or N atoms. NH 3 is the only molecule listed that meets this criterion.13 . C
The lone pair on the N atom in NH 3 gives it a triagonal pyramidal shape. Since the polar bonds
are not symmetric about the central atom, the molecule is polar. CH 4 and CO 2 contain polar
bonds, but the bonds are arranged symmetrically and their polarities cancel each other out;
these molecules are nonpolar. N 2 and O 2 do not have any polar bonds.
14 . D
In the Lewis structures of these molecules, only N 2 has a triple bond.
15 . E
Only the noble gases exist as individual gas-phase atoms under standard conditions. All other
substances form molecules, covalent networks, metallic solids, etc.
16 . A
The common allotropes of carbon are graphite and diamond, both of which are network
covalent solids.
17 . D
Fluorine has 7 valence electrons, and therefore can attain a noble gas configuration by gaining
one electron to form Fâ.
18 . E
Ionization energy is the energy required to remove an electron from the outer shell of an
atom. Ionization energy tends to increase from left to right across a row in the periodic table;
neon has the highest ionization energy of the elements in the second row.
19 . A
Reactions that decrease the number of moles of a gas, or that go from gas, liquid, or aqueous
phase reactants to solid phase products, generally have a decrease in entropy (âS < 0).
20 . B
Option B represents the combustion of a hydrocarbon (specifically, pentane). Combustion of a