Chemistry · Ch 7 — The p-Block Elements
Phosphorus — Allotropic Forms
Phosphorus — Allotropic Forms
Elemental phosphorus does not exist as a single form — it occurs as several allotropes, of which white, red and black phosphorus are the most important. Each differs sharply in structure, reactivity and physical appearance, even though all are built from the same phosphorus atoms.
White Phosphorus
White phosphorus is a soft, translucent, waxy solid. It is extremely poisonous, does not dissolve in water but dissolves readily in carbon disulphide, and — strikingly — glows faintly in the dark, a phenomenon called chemiluminescence.
Structurally, as the ball-and-stick diagram shows, white phosphorus consists of discrete tetrahedral molecules: four phosphorus atoms sit at the corners of a tetrahedron, joined by P–P bonds, with each P–P–P bond angle compressed to only . This is far below the angle a phosphorus atom would normally prefer, so the cage carries considerable angular strain. That strain is the reason white phosphorus is the least stable and most reactive of the solid allotropes under ordinary conditions.
Its reactivity shows up in two characteristic reactions:
- In an inert atmosphere, white phosphorus dissolves in boiling sodium hydroxide solution to liberate phosphine gas and sodium hypophosphite:
- Exposed to air, it ignites spontaneously, burning to give dense white fumes of phosphorus(V) oxide:
Red Phosphorus
Heating white phosphorus at 573 K in an inert atmosphere for several days converts it into red phosphorus. Unlike its parent form, red phosphorus has an iron-grey metallic lustre, is odourless and non-poisonous, and — notably — is insoluble both in water and in carbon disulphide. Chemically it is much more sluggish than white phosphorus, and it does not display the chemiluminescent glow.
The structural reason for this stability is shown in the chain diagram: red phosphorus is polymeric, built from many -derived tetrahedral cage units linked end to end into an extended chain rather than existing as isolated, strained molecules. Because the individual cages are joined into a network instead of standing alone under angular strain, red phosphorus is far less reactive than the white form.
Black Phosphorus
Black phosphorus is the most thermodynamically stable allotrope and exists in two forms. …
What this figure shows. Ball-and-stick tetrahedral P4 molecule. Four green spheres labelled P: one at the top vertex, one at bottom-left, one at bottom-right, and one at the front/bottom-centre vertex (drawn slightly forward, connected by dashed lines to show it sits behind/in front of the triangle plane). Solid blue-grey bond lines connect top-P to bottom-left-P and top-P to bottom-right-P and top-P to the front vertex; the bottom-left-to-bottom-right edge is also solid; edges to the front/back vertex partly shown dashed to indicate 3-D depth. A small arc between two of the bonds near the top vertex is labelled '60°', marking t …
What this figure shows. Polymeric chain drawing showing three tetrahedral P4-derived cage units linked left-to-right in a row. Each unit is a cluster of green spheres labelled P: one apex P at top and a base of P atoms below/around it, joined by dark blue bond lines forming a small cage/basket shape (similar cage motif repeated three times). Adjacent cage units are connected to each other by horizontal bond lines running between base P atoms of neighbouring units, and the chain has open/dangling bond lines at the extreme left and right ends (indicated by short line stubs) showing the chain continues indefinitely in bot …