Chemistry, Third edition

54 4 · BONDING BETWEEN ATOMS

Example 4.3 (continued)

 Comment

The structural formula of CCl 4 is written as

Cl

|

Cl—C—Cl

|

Cl

Writing Lewis structures

Write Lewis structures for the following covalent molecules, then write their structural

formulae using —— to represent a shared pair of electrons:

(i) H 2 (ii)CH 4 (iii)SiCl 4 (iv)F 2 (v)H 2 O (vi)OCl 2 (vii)PCl 3

(viii)ICl (iodine has seven electrons in the outer shell of its atoms)

(ix) HBr (x)CHCl 3 (C is the central atom).

Exercise 4D

Valency of an atom

Can you now see where we got the idea of ‘hooks’ to describe combining power, or

valency, in Chapter 2? The number of ‘hooks’ that an atom or group possesses is actually a

simple way of describing how many electrons that atom or group needs to give away, accept

or share in order to obtain the stable configuration of a noble gas. Carbon, for example,

has the electronic structure 2.4. We describe it as having a valency four, or four

‘hooks’; this is just another way of saying that it needs to share four more electrons to

achieve the stable electronic structure of neon, namely 2.8.

Multiple bonds

Sometimes, more than one pair of electrons is shared. Consider oxygen gas (O 2 ):

Here, each oxygen shares two pairsof electrons to achieve the electron configuration

of neon. The structural formula of this oxygen molecule is written

O===O

Adouble bondexists between the oxygen atoms. In a structure where three pairs of

electrons are shared, the bond is called a triple bond.

Covalent molecules with multiple bonds

Write Lewis structures for the following, then write their structural formulae:

(i) CO 2 (iv) CS 2

(ii)N 2 (v) H 2 CO (C is the central atom).

(iii)COCl 2 (C is the central atom)

Exercise 4E