Question

Why are diamonds different in appearance compared to graphite if they are both made of carbon atoms?

Answer 1

These are allotropes of the one element whose structures are markedly different.

Explanation:

Graphite is non-molecular in 2 dimensions. Graphite is an excellent lubricant inasmuch as the 2-dimensional carbon layers can slide over each other.

On the hand, the diamond structure is non-molecular in 3 dimensions. Each carbon atom is strongly bound to 4 other carbon atoms with a tetrahedral geometry, whose bonding extends indefinitely over a crystalline lattice.

This difference in structural properties gives rise to the physical difference between the carbon allotropes. Because of their non-molecularity, both graphite and diamond have exceptionally high melting points.