Aluminium

Aluminium is in group 3 and has an electronic configuration of

1s²2s²2p⁶3s²3p¹.

Hydrated Aluminium ions are acidic.

Aluminium has a very high ratio, since it has a charge of +3 and is found in period 3, thus making it a very small cation.

When Al³⁺ are dissolved in water, since Al³⁺ has a high polarising power due to the large , it will pull electron away from water, releasing H⁺ ions in solution, giving it a pH lower than 7.

Al₂O₃ is an amphoteric oxide

Al₂O₃ is an amphoteric oxide, thus reacting with both acids and bases.

Reaction with an acid:

Reaction with a base:

AlCl₃ 

Preparation of anhydrous AlCl₃:

Dry AlCl₃ cannot be prepared by drying hydrated AlCl₃ since this would result in the preparation of the oxide.

Dimerisation of AlCl₃

AlCl₃ is an electron-deficient compound, and therefore it can dimerise so that the aluminium can get 8 electrons in the outer shell. This is possible with the Cl donating two electrons and forming a dative bond with the Aluminium.

AlCl₃ can also sublime at 180^oC

Alum Salts

An alum  is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO₄)₂·12H₂O, where X is a monovalent cation such as potassium or ammonium. By itself, “alum” often refers to potassium alum, with the formula KAl(SO₄)₂·12H₂O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.

The name “alum” is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium(III). The most common of these analogs is chrome alum KCr(SO₄)₂·12H₂O