Question

______ is the crystalline allotrope of carbon.
(A) Coal
(B) Coke
(C) Diamond
(D) Graphite

Answer 1

Hint: Think about what the word ‘allotrope’ means. We keep saying the glass is amorphous and quartz is a crystal. Recollect the differences between both crystalline form and amorphous form. Take a look at all the options and find out the most suitable answer.

Complete step by step solution:
-An allotrope is a substance which exists in more than one physical form due to different arrangement of atoms of an element in space.
-Allotropes are two or more physical forms in which an element exists. This property is known as Allotropy.
-Generally, the compounds having the property of catenation exhibit this property of allotropy.
-For example, carbon forms different types of allotropes like coal, graphite, diamond, coke, etc.
-Allotropes are further classified into crystalline and amorphous forms.
-A crystalline substance has a regularly repeating arrangement of atoms over a long-range. That is, it has long-range order. These compounds are hard in nature and generally shiny or lustrous.
-An amorphous substance doesn’t have long-range repeating order of atoms over the long-range. It has short-range order. That’s why these compounds are soft and brittle or powdery.
-Let’s take a look at the options.
-Coal and coke are amorphous allotropes of carbon because they lack long-range order.
-Diamond is a perfectly crystalline substance. Diamond and graphite are crystalline allotropes of carbon.

Therefore, the answer is options (C) and (D).

Note: Remember crystalline substances have a long-range ordered arrangement and amorphous substances have a short-range ordered arrangement of atoms. Diamond, graphite, graphene and fullerene are crystalline allotropes of carbon. Coke and coal are amorphous allotropes of carbon.