Q.What are allotropes? Sketch the structure of two allotropes of carbon namely diamond and graphite. What is the impact of structure on physical properties of two allotropes?

Diamond: every carbon $sp^3$-hybridised, tetrahedrally bonded to 4 neighbours, forming a giant covalent 3-D lattice — extremely hard, high melting point, non-conductor (all valence electrons localised in bonds). Graphite: every carbon $sp^2$-hybridised, bonded to 3 neighbours in planar hexagonal sheets, sheets stacked and held by weak van der Waals forces, with one delocalised electron per carbon — s …

1. What allotropes are. Allotropes are different physical forms in which an element can exist in the same physical state, differing only in the arrangement of their constituent atoms — they show different physical properties but generally similar chemical behaviour. Carbon has several allotropes, of which diamond and graphite are the classic examples.

2. Structure of diamond.

   • Each carbon atom is $sp^3$ hybridised and forms four equivalent, strong covalent bonds (C–C $\approx$ 154 pm) to four neighbouring carbon atoms arranged tetrahedrally.
   • This bonding pattern extends indefinitely in three dimensions, producing a single giant, rigid covalent network (not discrete molecules).

   (sketch: a repeating tetrahedral C-C-C-C framework extending in all three directions, every vertex a carbon atom.)

3. Structure of graphite.

   • Each carbon atom is $sp^2$ hybridised and bonds to three neighbouring carbons, forming planar hexagonal (six-membered) rings that extend into large flat sheets.
   • The fourth valence electron of each carbon is delocalised over the sheet, contributing to a network of $\pi$ bonding within the layer.
   • These flat sheets are stacked one above another, separated by about 340 pm, and held together only by weak van der Waals forces.

   (sketch: parallel hexagonal-mesh sheets stacked with larger gaps between layers than the C–C bond length within a layer.)

4. Impact of structure on physical properties.

   • Hardness: Diamond is exceptionally hard because every atom is locked into the rigid 3-D covalent network; graphite is soft because its layers can easily slide over each other (weak interlayer forces). …

Define allotropy, then contrast the two structures point by point: hybridisation of carbon ($sp^3$ vs $sp^2$), bonding geometry (3-D tetrahedral network vs 2-D hexagonal sheets), and the inter-layer forces in graphite. Use the structural differences to explain each contrasting ph …

• Describing diamond and graphite as having "different chemical properties" — allotropes have the same chemical behaviour but different physical properties. …