Why is allotropy

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The three non-interconvertible allotropes of red selenium are called α-, β- and γ- and all contain corrugated rings, analogous to sulfur. There is also an amorphous form of red selenium. All red forms are electrically non-conductive and dissolve in. The red modification results from the reduction of selenite-containing solutions in the cold. Due to the similarity to sulfur, mixed sulfur-selenium eight-rings with one to five selenium atoms can also be represented.

The black modification is obtained by reduction in heat; it is divided into a glass-like and an amorphous form. In the glass-like form there are larger rings (up to 1,000 atoms) and chains, which show rubber-elastic properties above 50, with further increase in temperature the selenium becomes increasingly plastic. This form is structurally very similar to the structure of quenched sulfur melts. In the melt, selenium is black; there are rings of various sizes and polymeric chains. As the temperature rises, the viscosity decreases and rings increasingly form.

The thermodynamically stable modification is the metallic, gray allotrope. It arises from the other modifications above 100. According to its metallic properties, it is electrically conductive and insoluble in. Spiral parallel chains exist in the crystal. The spirals in the crystal all have the same sense of rotation and have three atoms per turn. The distances between the chains are smaller than the van der Waals distance, so there are also covalent bonding forces between the chains. A regular crystal is only a poor electrical conductor, but if it is contaminated with halides, its conductivity increases significantly. Such selenium is used to manufacture rectifiers and photocells.

If you evaporate selenium, you get a yellow vapor made up of - and smaller rings. Above 900 molecules are preferred which are paramagnetic and have a double bond.