Which pair are allotropes of the same element?

• A. Diamond and graphite
• B. Sodium chloride and water
• C. Amorphous silicon and crystal silicon
• D. Oxygen and hydrogen

Answer: (A)

Allotropy is when an element exists in more than one structural form in the same phase. Diamond and graphite are the classic example of this for carbon. In diamond, each carbon atom is bonded tetrahedrally to four others, forming a rigid three-dimensional network that makes diamond extremely hard. In graphite, carbon atoms bond in planar hexagonal sheets with strong in-plane bonds but weak between the layers, so the sheets slide easily and graphite conducts electricity along the planes. The two forms have the same element, but the different bonding arrangements give very different properties, showcasing allotropy.

The other options don’t fit because they involve compounds or different elements (sodium chloride is NaCl, a compound; water is H2O; oxygen and hydrogen are different elements). The pair of amorphous and crystalline silicon is another form of the same element, but the well-known and textbook example illustrating allotropy is diamond and graphite.