Allotropic forms of phosphorus

Phosphorus exhibits several allotropic forms, of which the three most common are white (yellow) phosphorus, red phosphorus and black phosphorus, along with less common scarlet and violet forms. Freshly prepared white phosphorus is colourless but rapidly turns pale yellow on standing as a thin surface layer of red phosphorus forms, which is why it is often called yellow phosphorus. It is highly poisonous, has a characteristic garlic-like odour, and glows in the dark owing to slow aerial oxidation — a phenomenon called phosphorescence. Because its ignition temperature is very low, white phosphorus undergoes spontaneous combustion in air at ordinary room temperature, burning to phosphorus pentoxide (P2O5); for this reason it is normally stored under water. Structurally, white phosphorus consists of discrete tetrahedral P4 molecules, in which each phosphorus atom is joined to the other three purely by single P-P bonds (there is no P≡P triple bonding, unlike N≡N in dinitrogen, since phosphorus atoms are too large for effective p-orbital overlap to sustain a strong triple bond; instead the single-bonded, strained 60° P-P-P tetrahedral arrangement is thermodynamically preferred for phosphorus). …