Question

give examples of allotropes of group 16 elements.

Answer 1

In group 16 elements, sulphur exists in numerous allotropic forms of which the following two are the most important:
1. Rhombic or $\alpha$-sulphur

It is the most stable form of sulphur at room temperature.
it is yellow in colour.
It consists of stacks of puckered and crown shaped S₈ rings. The S-S bond length is 204pm and S-S-S bond angle is 107°.
Crystals of rhombic sulphur are formed on evaporating the solution of roll sulphur in CS_2.
Melting point: 385.8 K
Specific gravity: 2.06
It is insoluble in water and partially soluble in benzene, alcohol and ether and readily soluble in CS₂.
It transforms to the monoclinic or $\beta$-form on heating above 369 K.

2. Monoclinic sulphur (β-sulphur)

Like rhombic form, it also contains 8 atoms in a puckered ring shaped like a crown. However, these rings are stacked in a different manner than rhombic sulphur.
It is prepared by melting rhombic form in a dish and cooling the solution till a crust is formed. The remaining liquid in the crust is removed and colourless needle shaped crystals of β-sulphur are formed.
It has a melting point of 393K
Specific gravity of monoclinic sulphur is 1.98.
It is soluble in CS₂.
It is stable above 369K and at temperature below this it transforms into the rhombic form.

3. Synthetic forms of sulphur
These molecules have been synthesised in the laboratory and the number of S atoms range from 6 to 20. Cyclo-S₆ is a six ring sulphur molecule with a chair conformation.

4. The diatomic gaseous form, S₂ is present at temperatures above 1000K and is paramagnetic.