White phosphorus 1: introduction

All of the allotropes of phosphorus are too reactive to be found uncombined in nature. White phosphorus is the most reactive of the allotropes, reacting spontaneously with oxygen in the air (it is commonly stored in water). Its name comes from the Greek for 'light bearer' due to the faint glow it emits on exposure to oxygen. (The word 'phosphorescence' is derived from this phenomenon, though this reaction of phosphorus with oxygen is more correctly called 'chemiluminescence'.) It's surprising that an element as reactive as phosphorus was discovered as early as 1669, when it was isolated from urine. Elemental phosphorus is used in safety matches and flares.

White phosphorus consists of P₄ molecules. The four phosphorus atoms are located at the vertices of a tetrahedron, as shown in the structure on the left. Given the symmetry of tetrahedra all four phosphorus atoms are in equivalent positions in the P₄ molecule. The bond angles are therefore 60°. One would expect a bond angle of 107°, as in ammonia, so there is considerable bond strain in the molecule. This is an important factor in the great reactivity of white phosphorus. The red and black allotropes of phosphorus are less strained and so less unstable.

Go to page 2 to look at the unit cell within the bulk structure of white phosphorus.

Page 2