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

Step 1. Allotropism is the existence of an element in more than one crystalline or molecular form while remaining in the same physical state; the individual forms are called allotropes. It is common across the p-block -- boron, silicon, germanium, tin, phosphorus, arsenic, antimony, oxygen, sulphur, selenium and tellurium all show it -- but carbon's set is the richest and most structurally striking.

Step 2. Diamond is built from sp³-hybridised carbon, each atom σ-bonded to four neighbours (C-C = 1.54 Å) in a tetrahedral 3-D lattice; with every valence electron locked into a bond, it is an electrical insulator and, being fully cross-linked in three dimensions, is extremely hard.

Step 3. Graphite is built from sp²-hybridised carbon in flat hexagonal sheets (C-C = 1.41 Å), each atom forming three σ bonds with the fourth electron delocalised as a mobile π system across the sheet -- giving graphite its electrical conductivity; sheets stack 3.40 Å apart, held only by weak van der Waals forces, so they slide easily, making graphite soft and a good lubricant.

Step 4. Fullerenes (e.g. C₆₀) are discrete, sp²-hybridised cage molecules with a fused hexagon/pentagon ring structure and a delocalised, aromatic π system; carbon nanotubes are graphite-like cylinders capped with fullerene ends, stronger than steel along their axis and conducting; graphene is a single, one-atom-thick sp² honeycomb sheet -- effectively an isolated layer of graphite.

Define allotropism generally, note its spread across the p-block, then contrast carbon's sp³ (diamond) and sp² (graphite/graphene/fullerene/nanotube) allotropes by structure and resulting property.

• Naming only diamond and graphite and stopping there -- 'specific reference to carbon' invites covering the full modern set (graphene, fullerenes, nanotubes) the chapter itself introduces alongside them.