Chemistry, Third edition

4 · BONDING BETWEEN ATOMS

Ionic compounds with polyatomic ions

Both ionic and covalent bonding exist within some compounds. Consider the deadly

poisonous compound potassium cyanide (KCN). This is an ionic compound; it con-

tains the ions K+and CN. A Lewis structure for the cyanide anion can be written as

The whole is written in square brackets with the negative charge outside to indi-

cate that the negative charge is spread across the ion and does not belong to one par-

ticular atom – this ‘spreading’ of the negative charge or delocalizationof the charge

makes the ion more stable. The atoms C and N in the cyanide ion achieve the stable

electronic arrangement of inert gases. They do this by covalent bonding withinthe

ion. The structural formula of the ion is written:

[CN]

Ions such as CNthat contain more than one atom are called polyatomic ions.

Other examples of polyatomic anions that form ionic compounds, but have cova-

lent bonding withinthe anion are hydroxide (OH), nitrate (NO 3 ), sulfate (SO 42 ),

carbonate (CO 32 ), hydrogencarbonate (HCO 3 ) and phosphate (PO 43 ).

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Bonding in polyatomic ions

Write Lewis structures for:

(i)ithe hydroxide ion, OHand

(ii)the phosphonium ion, PH 4 (P is the central atom).

Exercise 4K

Resonance structures

If you tried to write a structural formula for the carbonate ion you might have written:

Or perhaps you might have come up with:

Which one is correct? Actually, none of them is exactly correct. In cases like this the

Lewis model of bonding is not completely adequate.

It is possible for chemists to measure the lengths of the bonds in compounds.

Double bonds between atoms are shorter than single bonds between the same

atoms, so we might expect that measurement of the bond lengths in the carbonate

ion would reveal two long bonds and one short one. In fact, all the bonds are of the

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