Modern inorganic chemistry

134 GROUPS I AND II

ABNORMAL PROPERTIES OF LITHIUM AND

BERYLLIUM

As any group is descended the size of the atom and number of

electrons shielding the outer electrons from the nucleus increases

and the ionisation energy falls (see Table 6.2.)

Shielding of the outer electrons is least for the small lithium and

beryllium atoms and their ionisation energies are consequently

higher than other members of their respective groups. In the case

of beryllium the higher ionisation energy results in the bonding in

many beryllium compounds being covalent rather than ionic. (This

tendency is shown to a much lesser extent by magnesium which

forms some covalent compounds.)

The small lithium Li+ and beryllium Be2+ ions have high charge-

radius ratios and consequently exert particularly strong attractions

on other ions and on polar molecules. These attractions result in

both high lattice and hydration energies and it is these high energies

which account for many of the abnormal properties of the ionic

compounds of lithium and beryllium.

In view of the ionisation energies the electrode potentials for

lithium and beryllium might be expected to be higher than for

sodium and magnesium. In fact

Li + (aq) -f c" -> Lifs): F^ = -3.04V

Be^2 + (aq) -f 2e~ ->Be(s):£^= -1.85V

Ionisation energy refers to the process Li(g) - Li + (g) -f e~. whereas
the electrode potential measured in aqueous solution also includes
the energy of hydration of the Li^(g) ion once formed i.e. Li ""(§) 4-
xH 2 O — > Li^(aq). This hydration energy is large and in the case of
lithium compensates for the high ionisation energy. The value of
the second ionisation energy of beryllium (the energy to remove the
second electron) is so great that even the large hydration energy of
the Be^2 + cannot compensate for it, and E^~ is less negative.
The hydroxide of lithium, although soluble in water, is a weak
base owing to the great attraction between the Li^ and OH~ ions
(p. 74); the hydroxide of beryllium is really a neutral, insoluble
beryllium complex [Be(OH) 2 ] (p. 45).
L (H 2 0)J
When considering the fluorides, the high hydration energy of the
small fluoride ion, F", must also be considered (p. 78). The lattice
energy of beryllium fluoride is high but the combined hydration
energies of the Be2+ and F~ ions are sufficient for the BeF 2 to
dissolve, whilst the other fluorides of Group II elements having
lower M2+ hydration energy are insoluble in spite of lower lattice