128 GROUPS I AND II
As a consequence of the high ionisation energy of beryllium its
halides are essentially covalent, with comparatively low m.p., the
melts being non-conducting and (except beryllium fluoride) dis-
solving in many organic solvents.
The lower members in Group II form essentially ionic halides,
with magnesium having intermediate properties, and both mag-
nesium bromide and iodide dissolve in organic solvents.
The lattice energies of the Group II fluorides are generally
greater than those for the corresponding Group I fluorides; conse-
quently all but beryllium fluoride are insoluble. (The solubility of
beryllium fluoride is explained by the high hydration energy of the
beryllium ion, cf. LiF.) The high hydration energy of the Be2+ ion*
results in hydrolysis in neutral or alkaline aqueous solution; in this
reaction the beryllium halides closely resemble the aluminium
halides (another example of a diagonal relationship—p. 14).
The magnesium ion having a high hydration energy (Table 6.2)
also shows hydrolysis but to a lesser extent (than either Be2+ or
A13+). The chloride forms several hydrates which decompose on
heating to give a basic salt, a reaction most simply represented as
(cf.p.45):
MgCl 2 2H 2 O -> Mg(OH)Cl + HC1T+ H 2 O
Other Group II halides are essentially ionic and therefore have
relatively high m.p., the melts acting as conductors, and they are
soluble in water but not in organic solvents.
SUMMARY
Group I
Element Li Na K Rb Cs
Fluorides Insoluble Soluble
Heat of formation decreasing
Melting point decreasing
- Note that the Be2+ ion has a co-ordination number of 4 whereas most cations
have a co-ordination number of six. This is again the result of the very small size.
