Allotropes of phosphorous

White (left), red (two in the center), and violet phosphorous (right)

Phosophorus is an element that exists in different forms, called allotropes. The most common of these are white or red solids.

• In white phosphorous four atoms form a tetrahedral structure. There are two known crystalline forms. Because phosphorous will spontaneously ignite when it comes in contact with oxygen, it needs to be stored in a liquid such as water. Usually it is a waxy solid. Because it is highly flammable, white phosphorous is used as a weapon.
• Heating white phosphorous to about 270 °C (518 °F) without air will give red phosphorous. Red-phosphorous is amorphic, and will no longer self-ignite when exposed to air. It is used as a fire retardant. When expsed to a little energy, red phosphorous can explode.
• Black phosphorous is the most stable form. It is obtained by exposing white phosoprous to a high pressure.