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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Third Row Elements Know More

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Allotropic Forms of Sulphur and Phosphorus: Among the third row elements, sulphur and phosphorus exhibit allotropy. The allotropic forms of phosphorus are-white or yellow phosphorus, red phosphorus and black phosphorus. The allotropic forms of sulphur are - rhombic sulphur, monoclinic sulphur and plastic sulphur.

Phosphorus: White phosphorus is a soft waxy solid. It has a characteristic garlic smell. When it is exposed to air it turns yellow. It is poisonous in nature. It exists as tetra-atomic molecules. The molecule is tetrahedral in shape.
Red phosphorus is prepared by heating white phosphorus. It is non-poisonous. Tetrahedral units are held together to form a linear chain.
Black phosphorus is prepared by heating white phosphorus. Black phosphorus has a layer type structure.

Sulphur: Rhombic sulphur is also called a sulphur. It is bright yellow in colour. The sulphur molecule is octa-atomic (S8). It forms a puckered ring structure.
Monoclinic sulphur is also called as
b sulphur. The sulphur molecule (S8) exists as puckered ring structure. But the symmetry of monoclinic sulphur crystals is different from those of rhombic sulphur crystals.
Plastic sulphur exists as zig-zag chains.

Chemical Properties of White Phosphorus
It reacts with metals to form their corresponding phosphides.
It reacts with non-metals like oxygen to form oxides.
It has a low ignition temperature, hence it is kept immersed in water.
It reacts with halogens to give their halides.
It reacts with aqueous alkali to give phosphine gas and sodium hypophosphite.

Oxyacids of Phosphorus:
The various oxyacids of phosphorus are:
Hypophosphorus acid H3PO2
Phosphorus acid H3PO3
Hypophosphoric acid H4P2O6
Orthophosphoric acid H3PO4
Metaphosphoric acid (HPO3)n
Pyrophosphoric acid H4P2O7


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