Which pair are both allotropes of carbon?

Which pair are both allotropes of carbon?

• A. Graphite and oxygen
• B. Graphite and diamond
• C. Diamond and silicon
• D. Graphene and coal

Answer: (B)

Allotropes are different structural forms of the same element. Carbon can bond in several distinct ways, giving forms with very different properties. Graphite is made of carbon atoms arranged in flat sheets of hexagonal rings that stack; the bonds within the sheets are strong, but the sheets can slide past each other, making graphite opaque and able to conduct electricity along the planes. Diamond, in contrast, has carbon atoms linked in a three-dimensional tetrahedral network, which makes it exceedingly hard and an insulator. Because both forms contain only carbon but have different bonding patterns and structures, they are allotropes of carbon. The other options mix different elements or are not pure carbon forms: oxygen is a different element; silicon is a different element; coal is not a single pure form of carbon but a mixture of many substances. Therefore, graphite and diamond are the pair that are both allotropes of carbon.

Allotropes are different structural forms of the same element. Carbon can bond in several distinct ways, giving forms with very different properties. Graphite is made of carbon atoms arranged in flat sheets of hexagonal rings that stack; the bonds within the sheets are strong, but the sheets can slide past each other, making graphite opaque and able to conduct electricity along the planes. Diamond, in contrast, has carbon atoms linked in a three-dimensional tetrahedral network, which makes it exceedingly hard and an insulator. Because both forms contain only carbon but have different bonding patterns and structures, they are allotropes of carbon. The other options mix different elements or are not pure carbon forms: oxygen is a different element; silicon is a different element; coal is not a single pure form of carbon but a mixture of many substances. Therefore, graphite and diamond are the pair that are both allotropes of carbon.