The Foundations of Chemistry

Step 4: C already has an octet, so the remaining 8 eare distributed as lone pairs on

the S atoms to give each S an octet. The complete Lewis formula is

Check: 16 e(8 pairs) have been used. The bonding picture is similar to that of CO 2 ; this is not surprising,

because S is below O in Group VIA.

You should now work Exercise 29.

EXAMPLE 7-3 Writing Lewis Formulas

Write the Lewis formula for the carbonate ion, CO 32 .

Plan

The same stepwise procedure can be applied to ions. We must remember to adjust A, the total

number of electrons, to account for the charge shown on the ion.

Solution

O^2 

Step 1: The skeleton is O C O

Step 2: N 1    8 (for C) 3    8 (for O) 8  24  32 eneeded by all atoms

Step 2: A 1    4 (for C) 3    6 (for O) 2 (for the 2charge)

Step 2:  4  18  2  24 eavailable

Step 2: SNA 32 e 24 e 8 e(4 pairs) shared

O^2 

Step 3: OSChSSO (Four pairs are shared. At this point it doesn’t matter which O is doubly

bonded.)

Step 4: The Lewis formula is

Check: 24 e(12 pairs) have been used.

You should now work Exercises 30 and 38.

You should practice writing many Lewis formulas. A few common types of organic

compounds and their Lewis formulas are shown here. Each follows the octet rule. Methane,

CH 4 , is the simplest of a huge family of organic compounds called hydrocarbons(composed

solely of hydrogen and carbon). Ethane, C 2 H 6 , is another hydrocarbon that contains only

single bonds. Ethylene, C 2 H 4 , has a carbon–carbon double bond, and acetylene, C 2 H 2 ,

contains a carbon–carbon triple bond.

C

H

H

H C

H

H

C H H C CH

H

H

H H

methane, CH 4 ethane, C 2 H 6 ethylene, C 2 H 4 acetylene, C 2 H 2

C C

HH

H H

O O

OO

C

2 ^2 

C

O O

or

SSC or SSC

Calcium carbonate, CaCO 3 , occurs
in several forms in nature. The
mineral calcite forms very large clear
crystals (right); in microcrystalline
form it is the main constituent of
limestone (top left). Seashells (bottom)
are largely calcium carbonate.

286 CHAPTER 7: Chemical Bonding

Acetylene burns in oxygen with a
flame so hot that it is used to weld
metals.

For practice, apply the methods of this
section to write these Lewis formulas.