Phosphorus: Allotropic Forms (White and Red)

Phosphorus, unlike nitrogen, shows extensive allotropy -- it exists in several distinct

structural forms, of which white phosphorus and red phosphorus are the two prescribed

here for detailed comparison, and the difference between them is a textbook illustration of how

structure alone, with no change in the element's identity, can completely change chemical

reactivity.

White phosphorus consists of discrete $\text{P}_4$ tetrahedral molecules: four phosphorus

atoms sit at the corners of a regular tetrahedron, each bonded to the other three by single

covalent bonds, so every P-P-P bond angle inside the cage is a strained $60^\circ$, far below the

normal $\sim 100^\circ$ angle preferred by phosphorus's bonding orbitals. This severe angular

strain stores considerable extra energy in the molecule and makes white phosphorus highly

reactive: it is a soft, waxy, translucent solid that is spontaneously flammable in air

(igniting around $303\ \text{K}/30^\circ\text{C}$, burning with a characteristic greenish glow

called chemiluminescence, and producing dense white fumes of $\text{P}_4\text{O}_{10}$), and it

is also acutely poisonous. Because of this extreme air-sensitivity, white phosphorus is always

stored under water, in which it is insoluble, though it dissolves readily in non-polar solvents

such as carbon disulphide.

Red phosphorus is obtained by heating white phosphorus in an inert atmosphere at around

$573\ \text{K}$ ($300^\circ\text{C}$) for several hours. Structurally, red phosphorus is best

described as a polymeric solid: the strained $\text{P}_4$ tetrahedron is opened by breaking

one P-P bond, and the resulting P4 units link end-to-end into long chains of tetrahedra, which

pack together (with further cross-linking) into an extended, three-dimensional, iron-grey/red

network. Because it has no small, strained, discrete molecular units and no comparably weak

intermolecular packing, red phosphorus is far less reactive than white: it does not ignite

spontaneously in air (its ignition temperature is much higher, roughly $533\ \text{K}$), it is

non-poisonous, it does not glow in the dark, and it is insoluble in both water and carbon

disulphide. This dramatic difference in behaviour between two forms of the identical element, …