Allotropy

The four allotropes of phosphorus: white, red (the two most common), black, and violet

Allotropy is the tendency of substances to come in different allotropes. Allotropes are different physical forms of the same element. Phosphorus, for example, forms several allotropes. The most common ones are white phosphorus and red phosphorus. White phosphorus is a pale yellow waxy solid that is highly toxic. It spontaneously ignites in air and glows in the dark. It is made of P₄ molecules, or four phosphorus atoms in one white phosphorus molecule. White phosphorus can be heated in a vacuum to make red phosphorus. Red phosphorus is a reddish-purple crumbly solid that is relatively nontoxic. It burns in air, but at a high temperature. It is made of chains, or polymers, of P₄ molecules.