We know that matter can exist in three physical states, that is, liquid and gas. Water exists in all three states. But water cannot exist in different forms in the same state. Some elements like sulphur, phosphorus and carbon do so. These elements can exist in more than one form in the same physical state having the same chemical properties but different physical properties. These different forms are called allotropes. Sulphur has three allotropic forms – rhombic, monoclinic and plastic sulphur. Phosphorus has two – yellow phosphorus and red phosphorus. Carbon has three allotropes – diamond, graphite and amorphous carbon.
The property due to which an element exists in more than one form in the same phusical state, having the same chemical but different physical properties, is called allotropy. The phenomenon is called allotropism. Allotropism occurs due to the difference in the method of crystallization and the number and arrangement of atoms in a molecule.