What is allotropy?

Hint:The word allotropy is derived from two Greek words allos and tropos. In Greek, allos means other and tropos means form. The name itself gives us an idea about allotropy.

Complete answer:
In Greek the word allotropy means other forms. Chemical elements exist in different arrangements in a particular physical state. These different forms or arrangements are known as allotropes. The bonding patterns among the atoms or molecules are different in allotropes and hence, we can call them as the structural modifications of an element. The phenomenon of existing in different allotropic forms is allotropy. The term allotropy is used for referring only to elements, not compounds.
A common example to discuss allotropy is Carbon. We know carbon exists in different allotropic forms, namely diamond, graphite, fullerene, etc. We are familiar with the structures and properties of these allotropes and know that they differ in physical properties due to the difference in the arrangement and bonding of the carbon atoms. Chemically all the allotropic forms represent elemental carbon.

Note:
Allotropy is said to be shown if the element exists in different forms in the same phase. The same element is not considered as an allotrope in different phases. For example, if we consider water. Ice, water and steam are the three different phases in which water molecules exist and are not the allotropic forms of water. However, if we consider oxygen, it exists as either ozone or dioxygen which can both exist in different phases, namely soli, liquid and gas. They can be considered as allotropes as though the phases are different, the molecular formula is also different.