Chemistry, Third edition

COVALENT BONDING

its atoms to achieve the stable electronic arrangement of a noble gas. The structural

formulaof the molecule is written as

Cl——Cl

where ‘——’ represents a pair of shared electrons, i.e. a covalent bond.

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Example 4.2

Ammonia (NH 3 ) is a covalent substance. Draw a Lewis structure

to represent the bonding in ammonia.

Answer

Nitrogen, which has the electronic structure 2.5 shares its outer electrons with

three hydrogens (electron arrangement 1). In this way nitrogen, and the three

hydrogens, achieve the electronic structure of noble gases:

Comment

The structural formula of ammonia is written:

H—N—H

|

H

(remember, —— means ‘a pair of shared electrons’)

Example 4.3

Molecules of tetrachloromethane (often called carbon

tetrachloride) have the formula CCl 4. What is the Lewis structure

of the molecule?

Answer

Carbon has the electronic structure 2.4 and chlorine the electronic structure

2.8.7. One carbon atom shares electrons with four chlorine atoms so that all

atoms achieve the electronic structure of a noble gas: