Sulphur, the Element and Allotropes
Sulphur occurs naturally in the elemental form, and is also found in the gases H2S and SO2. Where oxygen occurs as the gas, sulphur is a solid under standard conditions. This is because it does not form S=S double bonds, but forms rings and chains characterized by S-S single bonds.
Sulphur rings Sn, (n=6-12,17,20) and chains
|The S-S distances and the S-S-S bond angles are constant, but torsional angles vary considerably.||S8|
The large number of forms of sulphur which may be present in a sample, and the ease of interconversion mean that temperature dependence of its viscosity is not regular.
Selenium and Tellurium are much less abundant, and Polonium has no stable isotopes. Selenium adopts structures with rings like sulphur, while Tellurium forms chains.
Catenation of Sulphur
Many H2Sn are known, with n up to 100, whereas H2O2 is thermodynamically unstable and strongly oxidizing.
Also, many polysulphides, Sn2-, are known.
The reason for the catenation is the greater bond strength of two S-S single bonds than one S=S double bond [2B(S-S)>B(S=S)]. This is the opposite of oxygen [B(O=O)>2B(O-O)].
This is a general trend in that on going down a group the tendency to form double bonds decreases.
Most metallic elements react directly with S, Se and Te: the reaction between mercury and sulphur occurs particularly readily. These binary compounds are known as chalcogenides.
Only group 1 and group 2 metals form ionic sulphides, and these are the only sulphides that dissolve in water.
Transition metal sulphides frequently have peculiar stoichiometries.
Transition metal sulphides with the Nickel Arsenide (NiAs) structure often have considerable metal-metal bonding and hence semimetallic character.