Allotropes of phosphorous and uses of phosphorus

Elemental phosphorus is obtained by heating phosphate rock with coke and silica in an electric furnace at about 1770 K. The phosphorus so formed is white phosphorus.

The reaction may be represented as:

formation of white phosphorus

There are three principal allotropic forms of Phosphorus. These are white phosphorus, red phosphorus and black phosphorus.

(a) White phosphorus or Yellow phosphorus

(i) It is a soft waxy solid with garlic smell.

(ii) It is poisonous in nature.

(iii) It turns yellowish on exposure to light. For this reason, it is also called yellow phosphorus.

(iv) It is not soluble in water but soluble in carbon disulphide.

(v) It undergoes spontaneous combustion in air and produces greenish glow.

(vi) It exists as P4 molecules both in solid and vapor state. The four atoms in P4 molecule occupy the corners of regular tetrahedron as shown in Fig. 7

(b) Red Phosphorus

It is prepared by heating white phosphorus to about 540 K in an inert atmosphere of nitrogen for several hours.

formation of red phosphorus from white phosphorus

(i) It is a hard crystalline solid without any smell.

(ii) It is non-poisonous in nature.

(iii) It is insoluble in water as well as in carbon disulphide.

(iv) It is more stable and relatively less reactive.

(v) It consists of tetrahedral units of P4 linked to one another to constitute linear chains.

(c) Black Phosphorus

It is prepared by heating white phosphorus to about 470 K under high pressure of 1200 atmospheres in inert atmosphere.

(i) It has metallic lustre.

(ii) It is most inactive form of phosphorus.

(iii) It has a layer type structure in which each layer consists of phosphorus atoms.

Some physical properties of three forms of Phosphorus are given below.

PropertyWhite phosphorusRed phosphorusBlack Phosphorous
ColourWhite, but turns yellow on exposureDark redBlack
ColourWhite, but turns yellow on exposureDark redBlack
StateWaxy solid, can be cut with knifeBrittle powderCrystalline with greasy touch
SmellGarlic smellOdorless
Density1.842.12.69
Ignition temperature307 K533 K673 K
Melting point317 KDoes not melt860 K

Uses of phosphorus
Phosphorus compounds play an important role in life processes. P is an essential constituent of animal and plant matter. It is present in bones, blood and brain of animal body and also in living cells. Several of its compounds have also industrial applications. The most important of these chemicals are orthophosphoric acid and phosphatic fertilizers.

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