The Foundations of Chemistry

Check: 16 pairs of ehave been used and all octets are satisfied. 2     16  32 e

available.

For NO 3 ,

Check: 12 pairs of ehave been used and all octets are satisfied. 2     12  24 e

available.

O

O O O

N



N

O

O

or



C is the central atom, or the element

in the middle of the molecule. It needs

four more electrons to acquire an

octet, and each S atom needs only two

more electrons.

7-5 The Octet Rule 285

EXAMPLE 7-1 Writing Lewis Formulas

Write the Lewis formula for the nitrogen molecule, N 2.

Plan

We follow the stepwise procedure that was just presented for writing Lewis formulas.

Solution

Step 1: The skeleton is N N.

Step 2: N 2    8  16 eneeded (total) by both atoms

Step 2: A 2    5  10 eavailable (total) for both atoms

Step 2: SNA 16 e 10 e 6 eshared

Step 3: NSSSN6 e(3 pairs) are shared; a triplebond.

Step 4: The additional 4 eare accounted for by a lone pair on each N. The complete

Lewis formula is

Check: 10 e(5 pairs) have been used.

EXAMPLE 7-2 Writing Lewis Formulas

Write the Lewis formula for carbon disulfide, CS 2 , a foul-smelling liquid.

Plan

Again, we follow the stepwise procedure to apply the relationship SNA.

Solution

Step 1: The skeleton is S C S.

Step 2: N 1    8 (for C) 2    8 (for S) 24 eneeded by all atoms

Step 2: A 1    4 (for C) 2    6 (for S) 16 eavailable

Step 2: SNA 24 e 16 e 8 eshared

Step 3: SSSCSSS8 e(4 pairs) are shared; two doublebonds.

N N or N N

Problem-Solving Tip:Be Careful of the Total Number of Electrons

A very common error in writing Lewis formulas is showing the wrong number of elec-

trons. Always make a final check to be sure that the Lewis formula you write shows the

same number of electrons you calculated as A.

For a slightly different approach

than the one used in this text,

see the Saunders Interactive General

Chemistry CD-ROM,Screen 9.5,

Drawing Lewis Structures.