Chemistry, Third edition

IONIC AND COVALENT COMPOUNDS – TWO EXTREMES

where ‘——’ represents a covalent bond, where one electron has been donated from

each atom, and ‘—’ represents a coordinate covalent bond, where both electrons

arise from one atom (in this case the nitrogen). In this way, all the atoms in the

molecule end up with the electronic configuration of an inert gas. There is no differ-

ence in an ‘ordinary covalent bond’ and a coordinate bond once they are formed (the

electrons no longer ‘belong’ to a particular atom taking part in the covalent bond) and the

above structure is often represented as

HF

||

H—N—B—F

||

HF

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Coordinate bonding

(i) Draw a Lewis structure for carbon monoxide (CO), bearing mind it contains a multiple bond

and a coordinate bond. Draw the structural formula of the molecule.

(ii)Draw a Lewis structure for the hydrated hydrogen ion, or hydronium ion H 3 O+.

(iii)Draw a Lewis structure for the ammonium ion, NH 4 +.

Exercise 4G

Ionic and covalent compounds – two

extremes

Although sodium chloride is regarded by chemists as an ionic compound and the

hydrogen molecule considered to be ‘completely covalent’, the vast majority of

chemical compounds fall between these two extremes. Water, for example, is

regarded as a covalent substance, and its molecules are neutral overall, but the

molecules do have a slight positive charge at one end and an equal negative charge at

the other end, i.e. there is a slight degree of ionic character. By the same token some

compounds which are generally regarded as ionic, such as calcium iodide (CaI 2 ),

have some covalent character.

Polar covalent molecules

The electrons in a bond between two identical atoms, such as that present in H 2 , can be

considered to be equally shared between both of them. If the atoms are not identical,

however, the electrons may be more attracted to one atom than the other. For example

in hydrogen fluoride, HF, the Lewis structure might be more accurately written as

Fluorine attracts electrons in a covalent bond much more than hydrogen – it has a

highelectronegativity. Because the shared electrons are more associated with the

fluorine atom rather than the hydrogen atom, the structural formula of the molecule

is sometimes written as

H—F

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