Which one of the following materials is not an allotrope of carbon ?

Allotropes are different physical forms in which an element can exist. Carbon exhibits several allotropes due to its unique bonding capabilities [2]. Diamond is a well-known crystalline allotrope where carbon atoms are arranged in a rigid tetrahedral structure, making it the hardest natural substance [2]. Graphene is a single-atom-thick sheet of carbon atoms arranged in a hexagonal lattice. Fullerenes, such as Buckminsterfullerene (C-60), consist of carbon atoms arranged in spherical or tubular shapes [1]. In contrast, fly ash is not an allotrope of carbon; it is a complex byproduct of coal combustion. Fly ash typically consists of inorganic oxides like silica, alumina, and calcium oxide, along with some unburnt carbon, rather than being a pure elemental form of carbon. Therefore, fly ash does not qualify as a carbon allotrope.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Table 3.1 > p. 40
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Allotropes of carbon > p. 61