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Boron Group (Group IIIA) Trends

Summary of Boron Group (Group IIIA) Trends:

1. The chemistry of boron is quite different from that of the heavier Group IIIA (Boron column) elements. It differs from aluminum in the following ways.

a) Its oxide and hydroxide are acidic, where as those of aluminum are amphoteric.

b) Boron is a semiconductor which has various polymorphs based on icosohedral boron cages, whereas aluminum is a metal with a close packed structure. Boron is very inert and only attacked by hot concentrated oxidizing acids.

c) No simple salts of B3+ are known, whereas those of Al3+ are numerous and well documented.

d) Boron forms a wide range of hydrides, which have cage structures. (AlH3)has a polymeric structure which resembles that of AlF3.

e) The stereochemistries of many boron compounds are based on trigonal sp2 and tetrahedral sp3 geometries. In the latter the octet rule is obeyed. Aluminum forms many compounds with tetrahedral, trigonal bipyramidal, and octahedral geometries.

f) Multiple pπ- pπ bonding in boron-nitrogen, boron-oxygen, and boron-fluorine compounds is more significant than for the corresponding aluminum compounds. {BN}x for example adopts a graphite structure.

2. Aluminum, gallium, indium and thallium all form a range of compounds in the +3 oxidation state and compounds in the +1 oxidation state become progressively more stable down group IIIA.

3. The oxides of aluminum and gallium are amphoteric and indium and thallium oxides are more basic.

4. The octahedral aqua-ions [M(OH2)6]3+ are acidic and the pKa values for the equillibria: [M(OH2)6]3+ ó [M(OH2)5(OH)]2+ + H+ are Al, ~5; Ga, 3; In, ~4; Tl, 1; showing the Al3+ aquo-ion is the least acidic and the Tl3+ ion the most acidic.

5. The MX3 compounds are Lewis acids and Lewis acid strengths decrease in the order: Al > Ga > In.

6. The stability of the hydrides decreases down the group and there are no stable Tl-H compounds. Extraordinary precautions are required to exclude air and moisture in order to isolate Ga2H6.

7. Aluminum is resistant to corrosion because of an impermeable oxide layer, but is soluble in non-oxidizing mineral acids. Gallium, indium, and thallium dissolve readily in acids, but thallium dissolves slowly in H2SO4 and HCl.

8. AlN, GaN, and InN have wurtzite structures analogous to cubic BN, but show no analogue of the graphite-type structure of BN.