Chemistry · Ch 9 — Elements of Group 13, 14 and 15
Allotropes of phosphorus
Allotropes of phosphorus
Phosphorus exists in several allotropic forms, of which white (or yellow) phosphorus and red phosphorus are the most important. White phosphorus is made of discrete, tetrahedral P4 molecules in which each P-P-P bond angle is compressed to 60° (see the White-P4 structure note); it is a translucent, waxy white solid that glows in the dark (chemiluminescence), is insoluble in water but dissolves in boiling NaOH solution, and is highly poisonous. Because the 60° bond angle in the P4 tetrahedron is far smaller than the roughly 109° angle unstrained P-P single bonds would prefer, the molecule carries considerable angular strain; this strain makes white phosphorus less thermodynamically stable, and therefore markedly more reactive, than red phosphorus. Red phosphorus is prepared by heating white phosphorus at 573 K in an inert atmosphere, which links the P4 tetrahedra together, through covalent bonds, into extended chains (see the Red-P-chain structure note); it is consequently a polymeric solid. Being polymeric and free of the angular strain of the discrete P4 unit, red …
What this figure shows. White phosphorus is drawn as a small, discrete tetrahedral cage of exactly four phosphorus atoms, one atom at each corner of the tetrahedron, with a P-P single bond running along each of the tetrahedron's six edges, so each P atom is bonded to the other three. The diagram marks the P-P-P bond angle at each corner as 60°, much smaller than the roughly 109° that an unstrained tetrahedral phosphorus centre would prefer, visually showing the origin of the angular strain that makes white phosphorus less stable and …
What this figure shows. Red phosphorus is drawn as a long, extended zig-zag chain built by linking many P4 tetrahedral units end-to-end through covalent P-P bonds, in contrast to white phosphorus's isolated, unconnected P4 tetrahedra. The repeating chain of linked tetrahedra shown in the diagram is what makes red phosphorus 'polymeric in nature', and this extended chain structure — free of the small-cage angular strain of white phosphorus — is consistent with red phosphorus's greater stabilit …