Chemistry, Third edition

4 · BONDING BETWEEN ATOMS

When ionic bonding is likely to occur

Ionic bonding occurs between elements when the energy required to remove the

outer shell electrons (the ionization energy) of one of those elements is relatively low.

Elements with such low ionization energies are metals; they generally contain no more

than three electronsin the outer shell of their atoms. Therefore, compounds contain-

ing a metal tend to be ionic. Non-metals generally contain more than three electrons

in the outer shells of their atoms. Note that, although hydrogen contains one electron

in the first shell of its atoms, the ionization energy for that atom is very high because it

is close to, and highly attracted to, the positive nucleus. Hydrogen is therefore classi-

fied as a non-metal and most of its compounds are not ionic.

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BOX 4.3

Hydrogen ion

For the process

H(g)H+(g) + e

the ionization energy is very high. Its ionization

energy is 1310 k J mol^1 (higher than the first

ionization energy for xenon at

1170 k J mol^1 !). The hydrogen ion can be

formed only when it is solvated. Solvation

occurs when the hydrogen ion attaches itself

to a solvent molecule; the energy released

when the hydrogen ion is solvated makes it

‘worthwhile’ ionizing hydrogen to H+. The most

common solvent with which to solvate H+is

water; the hydrogen ion in water should be

written as H 3 O+or H+(aq) after the process

H+(g) + H 2 O(l)H 3 O+

or H+(g)H 2 O(l)H(aq)

This process is known as hydrationand the

ion H 3 O+is commonly called the hydronium

ion. Remember that although you may see the

symbol H+, H+(aq) or a reference to the hydro-

gen ion, in water all of these terms refer to

H 3 O+.

Covalent bonding

What type of bonding occurs if it takes too much energy to remove electrons from

one of the atoms taking part in bonding? This situation occurs when two or more

non-metals combine – their atoms have to shareelectrons to achieve the stable elec-

tronic configuration of a noble gas. This type of bonding is called covalentbonding.

Consider chlorine gas; the Lewis symbol for chlorine is:

If two chlorine atoms share one electron, they achieve the stable electron arrange-

ment of argon. We can represent this on a diagram called a Lewis structure:

Amoleculeof chlorine is formed – there are no charged particles. The electrons of

one chlorine atom are usually represented by dots and those of the other by crosses,

to enable you to see where they come from. Remember, though, that once the electrons

are involved in the covalent bond there is no way to distinguish between them. Chlorine

gas therefore, chemical formula Cl 2 , exists as a diatomicmolecule because this allows

4.3