Question

Which of the following is a crystalline allotrope of carbon?
A.Charcoal
B.Coal
C.Fullerene
D.Lamp black

Answer 1

Allotropy is a characteristic property exhibited by some chemical elements which enables them to exist in more than one physical states. This means that the atoms of the elements can arrange themselves in different orientations and geometries to form different unit structures.

Complete Step-by-Step Answer:
Before we move forward with the solution for this question, let us first understand some basic important concepts.
Many a times, the structures thus formed in allotropy are non – uniform in nature and they do not have fixed geometries at the molecular level. If such compounds exist in the form of a solid, then they are known as amorphous solids. On the other hand, if these compounds have a fixed unit cell geometry, then they are known as crystalline solids.
The different types of geometries achieved by elements enables them to form compounds with completely different characteristics. Many a times, the structural integrity of the unit cell often defines the properties like tensile strength of the compounds so formed.
Out of the given options, charcoal, coal and lamp black are all amorphous solids. Hence, they do not have crystalline structures. On the other hand, fullerene exhibits a crystalline structure and has a molecular formula of ${C_{60}}$
Hence, Option C is the correct option

Note: you may tend to confuse allotropy and isomerism. Allotropy enables the elements to form different structures and geometries on the basis of the placement of the atom of the element alone. On the other hand, isomerism involves compounds with the same molecular formula to have completely different molecular structures.