SAT Subject Test Chemistry,10 edition

Resonance structures can differ only in the way the electrons are distributed; the arrangement of
the actual atoms cannot change.

Example: Write  the resonance   structures  for NCO–.

Solution:

formal  charges.

A   Lewis   structure   in  which   negative    formal  charges are placed  on  more    electronegative atoms

is  more    stable  than    one in  which   the formal  charges are placed  on  less    electronegative atoms.

C   is  the least   electronegative of  the three   given   atoms,  N,  C,  and O.  Therefore   the C   atom    occupies    the central

position    in  the skeletal    structure   of  NCO–.

N           C           O

1.

N   has 5   valence electrons;

C   has 4   valence electrons;

O   has 6   valence electrons;

and the species itself  has one negative    charge, indicating  the presence    of  an  extra   electron.

2.

Draw    single  bonds   between the central C   atom    and the surrounding atoms,  N   and O.  Place   a   pair    of  electrons   in

each    bond.

N   :   C   :   O

3.

4. Complete the octets  of  N   and O   with    the remaining   16  –   4   =   12  electrons.

The C   octet   is  incomplete. There   are three   ways    in  which   double  and triple  bonds   can be  formed  to  complete    the

C   octet:  two lone    pairs   from    the O   atom    can be  used    to  form    a   triple  bond    between the C   and O   atoms;

or  one lone    electron    pair    can be  taken   from    both    the O   and the N   atoms   to  form    two double  bonds,  one between

N   and C,  and the other   between O   and C;

5.