Which element is famous for existing in multiple crystalline forms such as diamond and graphite?

• A. Iron
• B. Carbon
• C. Oxygen
• D. Silicon

Answer: (B)

Allotropy in elements is the phenomenon where the same element can crystallize into different structures with very different properties. Carbon is famous for this. In diamond, carbon atoms form a rigid three-dimensional network where each atom bonds to four others, giving extreme hardness and a high melting point. In graphite, carbon atoms bond in two-dimensional sheets of hexagons with sp2 bonds; the sheets stack with weak interlayer forces, so the material is soft, can be lubricated, and conducts electricity along the planes. This stark contrast in properties arises from the different bonding and crystal arrangements of carbon atoms. Other options don’t pair diamond-like and graphite-like forms for the same element—iron has different crystal forms, oxygen mainly exists as O2 or O3 in gaseous/molecular forms, and silicon typically forms a diamond cubic structure rather than multiple distinct elemental allotropes like diamond and graphite.