13.4: The Elements

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A non-metal can be classified as an element that mostly lacks metallic attributes. Physically, non-metals tend to be highly volatile (easily vaporised), have low elasticity, and are good insulators of heat and electricity; chemically, they tend to have high ionization energy and electronegativity values, and gain or share electrons when they react with other elements or compounds. Seventeen elements are generally classified as nonmetals; most are gases (hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon and radon); one is a liquid (bromine); and a few are solids (carbon, phosphorus, sulfur, selenium, and iodine).

On moving across the periodic table, nonmetals are seen to adopt structures with progressively fewer nearest neighbours. Polyatomic nonmetals have structures with either three nearest neighbours, as is the case (for example) of carbon (in its standard state of graphite), or two nearest neighbours (for example) in the case of sulfur. Diatomic non-metals, such as hydrogen, have one nearest neighbour, and the monatomic noble gases, such as helium, have none. This gradual fall in the number of nearest neighbours is associated with a reduction in metallic character and an increase in nonmetallic character.

Allotropes are different structural forms of the same element in which changes in the connectivity of the covalent bonding between atoms results in substances with quite different chemical and/or physical properties. The change between allotropic forms is triggered by factors such as pressure, light, and temperature. Therefore the stability of a particular allotrope depends on particular conditions.

If covalent connectivity is the same but packing is different then you have polymorphs (eg. Monoclinic and Rhombic sulfur (S8) are polymorphs not different allotropes. S8 and S12 are different allotropes of S.

Many nonmetals have allotropes (that are less stable than their standard form) with either nonmetallic or metallic properties. Graphite, the standard state of carbon, has a lustrous appearance and is a fairly good electrical conductor. The diamond allotrope of carbon is nonmetallic, being translucent and having relatively poor electrical conductivity.

Catenation is the ability to form element-element bonded molecular networks.