Appearance
Boron is a non-metallic grey powder, and all the other memebers of the Group are soft, silvery metals. Thallium develops a bluish tinge on oxidation.

General Reactivity
The general trend down Group 3 is from non-metallic to metallic character. Boron is a non-metal with a covalent network structure. The other elements are much larger than Boron and are more ionic and metallic in character. Aluminium has a close-packed metallic structure but is on the borderline between ionic and covalent character in its compounds. The remainder of Group 3 are generally considered to be metals, although some compounds show covalent characteristics.

Occurrence and Extraction
These elements are not found free in nature, but are all present in various minerals or ores. Aluminium is the most widely used element in this Group. It is obtained by the electrolysis of Aluminium oxide, which is purified from Bauxite.

Physical Properties
The influence of the non-metallic character in this Group is reflected by the softness of the metals. The melting points of all the elements is high, but the melting point of Boron is much higher than that of Beryllium in Group 2, whereas the melting point of Aluminium is similar to that of Magnesium in Group 2. The densities of all the elements in Group 3 are higher than those in Group 2.

The ionic radii of the elements are much smaller than the atomic radii, as three outer electrons are lost in the formation of the ions. The resulting increased effective nuclear charge attracts the remaining electrons closer to the nucleus.

Chemical Properties
The chemical properties of the elements of Group 3 reflect the increasingly metallic characteristics of descending members of the Group. Only Boron and Aluminium will be considered here.

Boron is unreactive except at high temperatures. Alunminium 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 reduce strong alkali, a product being the tetrahydroxyaluminate ion, Al(OH)4-. Aluminium also reacts violently with Iron (III) oxide to produce Iron in the Thermit process:

2Al(s) + Fe2O3(s) è 2Fe(s) + Al2O3(s)


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

Hydrides
Boron forms an extensive series of Hydrides, the Boranes. The simplest of these is not BH3 as expected, but its dimer B2H6

Halides
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 terahydridoaluminate, LiAlH4.

Both Boron chloride and Aluminium chloride act as Lewis acids to a wide range of electron pair donors, and this has led to their widespread use as catalysts. Aluminium chloride is used in the important Friedel-Crafts reaction.