The existence of a substance and especially an element in two or more different forms (as of crystals) usually in the same phase is known as [[1]].

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Detailed Solution

Concept- The existence of a substance and especially an element in two or more different forms (as of crystals) usually in the same phase is known as allotropes. Allotropy is the property of some elements existing in more than one form. Example- There are 3 allotropes of carbon- graphite, diamond, and fullerene.
Allotropes are different forms of the same element showing different physical properties. Group 13-16 elements in the periodic table form allotropes. Apart from carbon, Phosphorus having atomic number 15 belongs to group 15 and has a minimum 12 allotropes.
In allotropes, atoms are arranged in different manners. This results in differences in their physical properties.
Phosphorus shows allotropy. The common forms are white, red and black phosphorus. The names are given according to their colour. In white phosphorus,

molecules are packed into a crystal structure. In red P, the atom has a chain structure. In black P, puckered layers of atoms are observed. Out of these three white P is most stable and black P is most stable.
Hence, the answer is allotropes.

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