Question

What are allotropes? Write the names of allotropes of phosphorus.

Answer 1

Hint: Allotropes are most prevalent in Group 13 to Group 16 of the modern periodic table. Some of the allotropes can even be more chemically stable than the element itself. Phosphorus belongs to the Group 15 of the modern periodic table.

Complete step by step solution:
Allotropes are two or more forms of the same element existing in the same physical state (either solid, liquid, or gas) that differ from each other in their physical, and sometimes also in chemical, properties. Allotropes is derived from the Ancient Greek word ἄλλος (allos), meaning 'other', and τρόπος (tropos), meaning 'manner, form'. This difference in physical properties displayed by allotropes of an element can be attributed to the fact that the atoms are arranged in different ways inside the molecules.

Phosphorus also shows the property of allotropy or allotropism. It exhibits several allotropic forms but the most common allotropes of are:
White Phosphorus
Red Phosphorus
Black Phosphorus

Let us compare all of these three allotropes.

Basis	White Phosphorus	Red Phosphorus	Black Phosphorus
Structure	P4 molecules packed into a crystal at a bond angle of 60 degree.	Chains of P4 molecules(polymer)	Puckered layers of phosphorus atoms(polymer)
Color	White	Red	Black
Stability	Least stable	Intermediately stable	Most stable
Toxicity	Highly toxic	Non Toxic and odorless	Non toxic

There are at least 12 allotropic forms of phosphorus have been reported. Violet (Hittorf’s) Phosphorus, Blue Phosphorus and DiPhosphorus also being few of them.

Note: Don’t confuse between allotropy and polymorphism. Allotropism is only the difference in the forms of pure chemical elements. Polymorphism is a phenomenon in which compounds display different crystalline forms.