GROUPS! AND II 121
FORMATION OF IONSWe note first that the elements are all electropositive, having
relatively low ionisation energies, and are, in consequence, very
reactive. The enthalpy change required for the process M(metal) -»
M + (g) for Group I, or M(metal) -> M2+(g) for Group II is at a
maximum at the top of each group, and it is, therefore, not surprising
to find that lithium, beryllium and, to some extent, magnesium do
form some covalent compounds. Most solid compounds of Group 1
and II elements, however, have ionic structures and the properties
associated with such structures—high m.p. and b.p., solubility in
water rather than in organic solvents and electrical conductance
when molten.
IONS IN SOLUTION
The hydration energies (strictly, hydration enthalpies) fall, as
expected, as we descend either Group, and are larger for Group II
than for Group I ions. The solubilities of the salts of Groups I and II
are determined by a balance between lattice energy, hydration
energy and the entropy change in going from solid to solution, and
only a few generalisations are possible. Thus high charge and low
ionic radii tend to produce insolubility (for example salts of lithium,
beryllium and magnesium, especially those with doubly charged
anions such as carbonate COa~). At the other end of the scale, low
charge and large radii also produce low solubility (for example salts
of potassium, rubidium and caesium containing large anions such
as the tetraphenylborate anion (p. 136). In between, solubility is the
rule for all Group I salts, and for most Group II salts containing
singly-charged negative ions; for many Group II salts with doubly-
or triply-charged anions (for example COj", SOj", PO^ ) in-
solubility is often observed.
The decreasing tendency to form salts with water of crystallisation
(as a group is descended) is again in line with the falling hydration
energy. For example, both sodium sulphate and carbonate form
hydrates but neither of the corresponding potassium salts do; the
sulphates of Group II elements show a similar trend MgSO 4 , 7H 2 O,
CaSO 4. 2H 2 O, BaSO 4. For the most part, however, the chemistry
of the Group I and II elements is that of the metal and the ions M +
for Group I and M^2 * for Group II. As already noted the two head
elements, lithium and beryllium, tend to form covalent compounds;
the beryllium ion Be^2 + , because of its very small radius and double