Allotropes

1. (b)

Crystals of rhombic sulfur (a); crystal of monoclinic sulfur (b)

Sulfur. Sulfur forms several allotropes.

Rhombic (-sulfur). It is of yellow colour,  = 2.07 g/сm³; stable at ordinary conditions and up to 95.4 ^оС (at fast heating), t_melting = 112.80 ^оС, insoluble in water, but it dissolves well in carbon disulfide.

Monoclinic (- and γ- sulfur). It is formed at heating to t > 95.4 ^оС from -sulfur. Has a primrose colour ( = 1.96 g/сm³, t_melting =119.3 ^оС).

-, -, and γ- allotropes are molecular structures with cyclic molecules S₈ in the sites of crystal lattice. The difference is the mutual positions of S₈ in a crystal.

Packing of - S₈ along b axis of unit cell in crystal lattice (a), β- S₈ along b axis (b), γ- S₈ along c axis (c)

-sulfur. At temperature 119,3⁰ sulfur melts, forming a mobile liquid of straw-colour. Its mobility is explained by the fact that between the nonpolar molecules S₈ act only weak dispersion forces which does not hinder the mutual moving of molecules.

-sulfur. At heating (~ >160⁰) liquid sulfur darkles and transforms into umber resinous mass. Its viscosity grows substantially (at a 190⁰ viscosity becomes approximately by 9000 times greater than at 160⁰). It is explained by the fact that at t>160⁰ the cyclic molecules of the sulfur S₈ begin to rupture and form longer chains (at a 171⁰ average length of liquid sulfur chain equals 1.5^.10⁶ bonds). Such very large molecules interplace between one another¹, as a result a liquid becomes very viscous.

Plastic sulfur allotrope

At higher temperatures (t>200⁰) viscosity of fusion of sulfur falls and at t>300⁰ it becomes an umber, mobile liquid again, which is explained by rapid thermal destruction of large macromolecular chains with gradual formation of shorter chains.

Plastic (amorphous) sulfur. It forms, if molten sulfur keeps for some time at 250^оС, and then is poured out into cold water. Plastic sulfur gradually transforms into rhombic (-sulfur).

Colloidal sulfur is produced by careful addition of acid to sodium thiosulfate solution.

S elenium. Among several allotropes of selenium, three nonmetallic ones contain Se₈ rings, whereas at least two others (gray metallic and amorphous red selenium) are semiconductors.

The most stable is hexagonal (grey) selenium. It consists of helical polymeric chains Se_, in which every atom is bound with two neighbours by the chemical bonds of 232 pm length. Chains are located in parallel with dispersion forces acting between them. Grey Se is a semiconductor. Its conductivity is light sensitive (by about 1000 times). Selenium vapours have the following equilibrium:

Sе₈ Sе₆ Sе₄ Sе₂ Sе

between Se_n (n=8-1) molecules, that is similar to sulfur, but due to the lower strength of bonds in Se_n equilibrium is shifted to the right.

The fast cooling of molten Se (for instance, pouring it in Н₂О) gives glassy allotrope of Se of red-brown color. It consists of disordered Se_ molecules of different length.

At a contact with certain organic solvents (СS₂), glassy selenium is slowly transformed to red crystalline Se consisting of cyclic Se molecules and having insulator properties. Red selenium dissolves slightly in СS₂ (about 0.05% at normal conditions). Boiling down² such solution below 72C, monoclinic -Se with the density of 4.5 g/сm³, and at higher temperature, cubic -Se (density of 4.4 g/сm³) can be isolated.

Elemental selenium produced in chemical reactions invariably appears as the amorphous red form: an insoluble, brick-red powder; for example, when obtaining Se by the action of SO₂ on SeO₂.

At 100-150⁰ all the mentioned above allotropes of selenium are transformed to the most stable gray Se.