The allotropes of Sulphur are interconvertible i.e. rhombic Sulphur when heated above $369K$, gives monoclinic Sulphur. However, the most stable form of Sulphur is rhombic Sulphur. Both rhombic and monoclinic Sulphur have the molecular formula ${S_8}$.

What is rhombic sulphur?

Rhombic sulphur is a crystalline allotropic form of sulphur. It is also designated as α–sulphur. This is the most stable variety of sulphur. All other varieties of sulphur eventually revert to rhombic form on standing.

Is S8 monoclinic or rhombic?

Both rhombic and monoclinic sulphur exist as S8 but oxygen exists as O2 .