Metals and Non-Metals - Part XVIII

Phosphorus
Phosphorus has chemical symbol P. It has 15 protons and 16 neutrons. An atom of P is represented as ³¹₁₅P. Phosphorous is a non-metal and exists in many allotropic[1] forms. Two allotropic forms commonly seen are: red phosphorus and yellow (or white) phosphorus. Phosphorus is indispensable to all living organisms; it is needed in bones, it is used for converting food into useful form of energy.

Since P has 15 protons, it also has 15 electrons. The electronic configuration of P is K-shell – 2 electrons, L-shell – 8 electrons and M-shell has 5 electrons. P is placed in Group VA of the periodic table, just after silicon, and below nitrogen. The reaction of P is similar to nitrogen. Sometimes phosphorus can have hybridized orbits similar to carbon and it forms covalent bonds.  

Finely powdered rock phosphate is mixed with coke and sand. This is then heated in an electric furnace to a temperature of 1000 to 1500°C. 

The reactions are in a two step processes and are shown below.  

The phosphorus obtained by this method is white or yellow phosphorus.

Physical properties of phosphorus : P  exists in many allotropic forms. Red and yellow phosphorus are two most commonly found allotropes of P. Other forms are purple and black phosphorus.

Since P has 5 electrons in the outermost shell, it borrows 3 electrons to form compounds. Phosphorus exists as P₄ and the structure is shown below. The bonds are covalent in nature and the bond angles are 60°.  

In case of red phosphorus, the basic structure is same but the total crystal is a complex structure of many P₄ molecules together. Thus bonding in red phosphorus is much stronger than in yellow or white phosphorus.

White phosphorus is obtained by the reactions mentioned above. This is a whitish, waxy soft solid. When exposed to light white P converts to red P. This reddish surface with white P together give a yellowish appearance to this type of P. The material is so soft that it can be cut by a knife. Its density is low (1.8 gm/cc). This P has a very low melting point of about 44°C. White P catches fire easily and its ignition temperature is about 30°C. Thus if white P has to be melted, it has to be held under water so that the material does not ignite. In the absence of air, white P boils at 290°C. White P is insoluble in water but soluble in carbon disulphide CS₂. Since white P catches fire easily it has to be stored carefully under water. White P is unstable and converts to its more stable allotrope, the red P. When white P can be heated to 250°C in an inert atmosphere (so that it doesn’t catch fire) of nitrogen or carbon dioxide, it converts to red P. White P smells like garlic.

Red Phosphorus is prepared from white P as discussed above. It is a dark powder. Its density is higher than white P (2.3 gm/cc). Red P is a very stable structure. It is insoluble in water and also in carbon disulphide CS₂. It is an odourless powder. Red P does not melt but sublimes when heated in the absence of air at about 400°C. Red P does not catch fire easily as its ignition temperature is 260°C.  Red P can be converted into white P when heated and the vapours condensed quickly in water.

Chemical properties of phosphorus (both the allotropes will have same chemical properties) :
1. Valence : Since P has 5 electrons in its M-shell. The M-shell is more than half filled. Hence P does not give off its electrons easily. It readily forms covalent bonds to complete its M-shell. It shows variable valence of 3 or 5. P is quite a reactive element and forms oxides, chlorides and sulphides readily.

2. Action of oxygen :  White or yellow phosphorus reacts with oxygen vigorously and catches fire easily. It forms phosphorus trioxide and phosphorus pentaoxide.  

Red phosphorus does not react with oxygen until it is heated to about 260°C. The reaction is same as shown above. This shows that white phosphorus is more reactive than its red allotrope. The reason being that the bonding in red P is stronger than white P.

3. Phosphorescence : White P when kept in dark glows. This is known as phosphorescence. This phenomenon occurs because white P burns in oxygen.  
Red P does not show phosphorescence.
White P also shows an unusual phenomena of cold flame. When kept under water and heated, white P vapourizes slowly and starts glowing. The temperature of this flame is low and hence this called as a cold flame.  

4. Action of alkalis : P reacts with concentrated NaOH, KOH to form phosphine gas. The reaction takes place in an inert atmosphere.

Phosphine is a highly inflammable and poisonous gas. When phosphine comes in contact with air it catches fire.
Red P does not react with NaOH or KOH, until it is heated to a very high temperature.

5. Reaction with chlorine : White P reacts with Cl to produce P- tetra chloride or P-penta chloride. The reactions are shown below.  

Red P reacts more slowly with Cl and the reactions are same as above.

6. Reaction with metals : White P reacts with metals like Na, Mg, Fe directly to give metal-phosphide. One such reaction is as follows :  

Red P reacts with metals only after it is heated strongly.

7. Reducing properties : White P reacts with oxygen very easily and hence it can be used as a reducing agent. Its reaction with nitric acid is shown below. The reaction shows how nitric acid is reduced and nitrogen dioxide is released.

Comparison between white P and red P

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