Allotropes are forms of an element that have the same physical state, but different chemical states. In other words, atoms may bond in a slightly different manner, which will provide a different structure to the element. The most common example of allotropes are those of Carbon. Carbon can exist in the form of graphite, diamond, fullerenes and graphene. Similarly, oxygen can exist as dioxygen (the commonly known O2 molecule) and as ozone (O3).
Allotropes of an elements have different physical and chemical properties. For example, diamond is the hardest known naturally occurring substance and has a very high melting point. Diamond has clear and colorless crystals and is a poor conductor of electricity. In comparison, graphite is dark and waxy in appearance, is a moderate conductor of electricity and commonly used in pencils (as lead).