Answers and Explanations

REVIEW QUESTIONS

1 . The H—F bond    is  more    polar   than    an  H—H bond.

“Polar” denotes unequal sharing of  electrons;  H—H must    have    equal   sharing,    since   the two

atoms   are the same.   H   and F   are different   atoms,  with    different   electronegativities,    and so  the

electrons   are unequally   shared.

2 . Cl 2     <  HCN <   NaCl

Cl 2    is  the least   polar,  because it  contains    two identical   atoms   that    must    share   electrons

equally.    HCN is  a   linear  molecule    with    a   triple  bond    between C   and N;  it  has a   dipole  moment

pointing    from    the relatively  electropositive H   atom    toward  the rather  electronegative N   atom.

Still,  we  would   expect  HCN to  be  less    polar   than    NaCl;   the bond    between Na  and Cl, a   metal

and a   nonmetal    whose   electronegativities differ  greatly,    is  completely  ionic.

3 .  C 2 H 4

C 2 H 4     has greater bond    distance    because it  is  a   double  bond,   and is  therefore   held    less

closely than    C 2 H 2 ,   which   is  a   triple  bond.

A.

C 2 H 2

C 2 H 2     has a   greater bond    energy  because it  is  a   triple  bond,   and more    energy  is  needed  to

break   it.

B.

4 .  I  and IV

Choices I   and IV  are both    correct Lewis   structures  and are,    in  fact,   equivalent. Choice  II

has an  impossible  configuration;  H   can never   be  double-bonded   to  anything,   since   the

maximum number  of  electrons   it  may possess is  two.    Choice  III is  also    impossible, since

having  four    bonds   around  H   would   imply   eight   electrons.

A.