315Allotropes Described Section
Aluminum- Post-transition Metal
Aluminum is ordinarily classified as a metal, given its luster, malleability and ductility, high
electrical and thermal conductivity and close-packed crystalline structure.
It does however have some properties that are unusual for a metal. Taken together, these
properties are sometimes used as a basis to classify aluminum as a metalloid:
Its crystalline structure shows some evidence of directional bonding.
Although it forms an Al3+ cation in some
compounds, it bonds covalently in most others.
Its oxide is amphoteric, and a conditional
glass-former.
it forms anionic aluminates, such behavior
being considered nonmetallic in character.
Stott labels aluminum as weak metal. It has the
physical properties of a good metal but some of the
chemical properties of a nonmetal. Steele notes the
somewhat paradoxical chemical behavior of aluminum.
It resembles a weak metal with its amphoteric oxide and the covalent character of many
of its compounds. Yet it is also a strongly electropositive metal, with a high negative
electrode potential.
The notion of aluminum as a metalloid is sometimes disputed given it has many metallic
properties. Aluminum is therefore argued to be an exception to the mnemonic that
elements adjacent to the metal-nonmetal dividing line are metalloids.
Aluminum Sulfate
Aluminum sulfate, alternatively spelt either aluminum or sulfate, is a chemical compound
with the formula Al 2 (SO 4 ) 3.
Aluminum sulfate is mainly used as a flocculating agent in the purification of drinking water
and waste water treatment plants, and also in paper manufacturing.
Aluminum sulfate is sometimes referred to as a type of alum. Alums are a class of related
compounds typified by AB(SO 4 ) 2 .12H 2 O. The anhydrous form occurs naturally as a rare
mineral millosevichite, found e.g. in volcanic environments and on burning coal-mining
waste dumps.
Aluminum sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number
of different hydrates, of which the hexadecahydrate Al 2 (SO 4 ) 3 •16H 2 O and
octadecahydrate Al 2 (SO 4 ) 3 •18H 2 O are the most common. The heptadecahydrate, whose
formula can be written as [Al(H 2 O) 6 ] 2 (SO 4 ) 3 •5H 2 O, occurs naturally as the mineral
alunogen.
The compound decomposes to γ−alumina and sulfur trioxide when heated between 580
and 900 °C. It combines with water forming hydrated salts of various compositions.