Modern inorganic chemistry

152 THE ELEMENTS OF GROUP III

The structure of these solid compounds is not known with certainty
but an approximate formula might be NaAlO 2 .xH 2 O. Many
aluminates occur in minerals, for example the spinels of general
formula MU(A1O 2 ) 2 where M may be Mg, Zn or Fe: these have a
mixed oxide structure, i.e. consist essentially of M^2 +. Al^3 + and O^2 ~
ions.

THE HALIDES

Boron and aluminium halides show many similarities but also

surprising differences. Table 7.2 gives the melting and boiling points

of the MX 3 halides.

Table 7.2

M.P, AND B.P. OF BORON AND ALUMINIUM HALIDES

Fluoride

Chloride

Bromide

Iodide

boron

144

166

227

323

m.p.(K)

aluminium

1530 (sublimes)

453 (2 atm.)

371

453

boron

174

285

364

483

fe.p.(K)

aluminium

1530 (sublimes)

453 (sublimes)

528

654

Boron halides are all covalently bonded with melting and boiling
points increasing as expected with the increasing molecular weight.
All boron trihalides exist as monomers in the vapour state and have
regular trigonal planar configurations. They are electron-deficient
compounds since in each halide the boron atom has only six
electrons in its second quantum level and consequently they are
electron pair acceptor molecules, i.e. Lewis acids. The ready hydro-
lysis of all the boron halides probably begins with the formation of
a coordination compound with water, the oxygen atom donating a
pair of electrons; this is rapidly followed by loss of hydrogen
chloride, this process continuing to give finally B(OH) 3 , i.e. boric acid.

Cl Cl H Cl OH 2 Cl

\ / / \ / I

B + O -» B -> B

Cl H Cl Cl Cl OH

+ HC1