Explain in detail about the Allotropy

Allotropy refers only to dissimilar forms of an element within the similar phase or state of matter (i.e. different solid, liquid or gas forms) - the changes of state among solid, liquid and gas in themselves are not considered allotropy. For some elements, allotropes have dissimilar molecular formulae which can persist in dissimilar phases - for example, the two allotropes of oxygen (dioxygen, O2 and  ozone, O3), can both exist in the solid, liquid and gaseous states. Conversely, some elements do not maintain distinct allotropes in dissimilar phases: for example phosphorus has numerous solid allotropes, which all reply to the similar P4 form when melted to the liquid state.