Q.Describe briefly allotropism in p- block elements with specific reference to carbon.

Allotropism is the ability of a single element to exist in more than one distinct crystalline or molecular form while remaining in the same physical state -- the different forms are called allotropes. Carbon's diamond and graphite are the textbook example: identical atoms, radically different structures and properties.

Allotropism is common across the p-block. Boron shows several allotropes (amorphous boron and various rhombohedral and tetragonal crystalline forms). Carbon's allotropes -- diamond, graphite, graphene, fullerenes and carbon nanotubes -- are covered in structural depth in the carbon-allotropes concept below. Silicon exists as amorphous and crystalline forms; germanium and tin each have two or more distinct allotropes (tin's grey, white, rhombic and sigma forms being a particularly well-known case of a metal changing allotrope, and even crumbling, with temperature). Phosphorus shows an especially rich set -- white, red, scarlet, violet and black -- and arsenic and antimony each show three colour forms. Oxygen exists as ordinary dioxygen and as ozone, while sulphur, selenium and tellurium each show multiple crystalline forms too. Taken together, allotropism turns out to be the rule rather than the exception across the non-metallic and metalloid stretch of the p-block.