Experimental Data

General Info

Ionisation energies

General Reactivity

The general trend down Group 13 is from non-metallic to metallic character. Boron is a non-metal with a covalent network structure. Because the other elements are larger than boron they display more ionic and metallic character. Aluminium is on the borderline between ionic and covalent character in its compounds despite having a close-packed metallic structure, the rest of Group 13 is generally metallic, but some compounds do show some covalent character.

Physical Properties

The densities of the group 13 elements are all higher than those in group 2 following the trend along the periods. The softness in the metals is caused by the non-metallic character throughout group 13. Group 13 elements boast high melting points and generally follow the trend. Boron has a higher melting point than beryllium in group 2, while aluminium's is similar to that of magnesium. The ionic radii are much smaller than the atomic radii. This is because the atom contains three electrons in a quantum level relatively far from the nucleus, and when they are removed to form the ion the remaining electrons are in levels closer to the nucleus. In addition, the increased effective nuclear charge attracts the electrons towards the nucleus and decreases the size of the ion.

Chemical Properties

The chemical properties of group 13 elements reflect the increasingly metallic character of descending members of the Group. I only managed to find info on boron and aluminium.

Boron is chemically unreactive except at high temperatures.

Aluminium is a highly reactive metal which is readily oxidised in air. This oxide coating is resistant to acids but is moderately soluble in alkalis. Aluminium can therefore reduce strong alkalis, producing Al(OH)₄. Aluminium also reacts violently with iron(Ill) oxide to produce iron in the Thermit process:

2Al(s) + Fe₂O₃(s) ==> 2Fe(l) + Al₂O₃(s)

Occurrence and Extraction

The group 13 elements are not found naturally, but can all be found in various minerals and ores. Aluminium is found in bauxite and cryolite and is the most widely used element in the group. Aluminium oxide is purified from bauxite then electrolysis is used to obtain aluminium. The melting point of the aluminium oxide is too high for electrolysis of the melt, so instead it is dissolved in molten cryolite.

Hydride Compounds

Boron forms an extensive and interesting series of hydrides, the boranes. The simplest of these is not BH₃ as expected, but its dimer B₂H₆.

Oxidation States and Ionisation Energies

Boron and aluminium occur only with oxidation number +3 in their compounds, and with a few exceptions their compounds are best described as ionic. The electron configuration shows three electrons outside a noble gas configuration, two in an s shell and one in a p shell. The outermost p electron is easy to remove as it is furthest from the nucleus and well shielded from the effective nuclear charge. The next two s electrons are also relativeIy easy to remove. Removal of any further electrons disturbs a filled quantum shell so is difficult. This is reflected in the ionisation energies. The first three ionisation energies are low, and the fourth very much higher.

Oxide Compounds

Boron oxide, B₂O₃, is an acidic oxide and an insoluble white solid with a very high boiling point (over 2000K) because of its extended covalently-bonded network structure. Aluminium oxide, Al₂O₃, is amphoteric.

Halide Compounds

The most important halide of boron is boron trifluoride, which is a gas. Aluminium chloride, AlCl₃, is a volatile solid which sublimes at 458K. The vapour formed on sublimation consists of an equilibrium mixture of monomers (AlCl₃) and dimers (Al₂Cl₆). It is used to prepare the powerful and versatile reducing agent lithium tetrahydridoaluminate, LiAlH₄. Both boron chloride and aluminium chloride act as Lewis acids to a wide range of electron-pair donors, and this has led to a common use as catalysts. Aluminium chloride is used in the Friedel-Crafts reaction.