The property by virtue of which an element exists in more than one form in the same physical state, having same chemical properties but different physical properties is called allotropy. The different forms are called allotropes. For example, carbon has three common allotropes: diamond, graphite and fullerenes.
Some elements show allotropy due to the following reasons:
Firstly, the methods of formation or preparation of the element are different. Secondly, the arrangement of atoms in a given form changes according to its method of formation or preparation. Thirdly, the element has different energies in different forms. Lastly, the conditions like pressure, light and temperature are different at the time of formation or preparation of the element which gives rise to different forms.