Question

Monoclinic sulphur has a melting point of 393K but it form rhobic sulphur in 369K .why ?

Answer 1

369K is called transition temperature because both the allotropes of sulphur are stable at this temperature. In other words, we can conclude that α sulphur(rhombic) is stable below 369K and it becomes β-sulphur(Monoclinic) above that temperature.

Rhombic and monoclinic sulphur, both have S₈ molecules. The alternative packing of S₈ molecules gives different crystal structures.