Question

why sulphur exists as polyatomic molecule (s8)

Answer 1

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This question arises because we expect Sulfur to behave like O2, N2, Cl2 and others. Due to their special characteristics Sulfur (and phosphorous also ) behave differently. Probably because Sulfur forms single bonds rather than double bonds as an element. 

It exists as disulfide but as an ion with two negative charges, not as an element.

Oxygen does not have strong Vander Wall forces among atoms and so it exists as a gas at room temperature. It forms Pπ bonds.

Sulfur (and Selenium also in the same group) due to its bigger atomic size has stronger Vander Waal forces among atoms. So it forms a solid structure at room temperature. However, it forms Pπ-Pπ bonds but not so stable as oxygen. It is because of its size.

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Sulfur exists in many allotropic forms. S₈ is the ocagonal crystalline (cyclic or ring) structure with chemical bonds among the atoms.   It is best known as well as most stable allotrope at room temperature and normal atmosphere.  It reduces only in presence of strong acids. It is also known as ortho rhombic sulfur. Sulfur exists as 4 isotopes and they contain sulfur of different oxidation states.
S₇, S₆ also exist. 
Its electronic configuration is 1s² 2s², 2p⁶ 3s² 3p⁴.  It is similar to Oxygen which exists as O₂.

Sulfur seems to exist as disulphide  ion : as   S₂ ²⁻    in many compounds like Fe S₂ (iron sulphide).

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Answer 2

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Because it's atomicity is 8

because Sulfur forms single bonds rather than double bonds as an element. It exists as disulfide but as an ion with two negative charges, not as an element. Oxygen does not have strong Vander Wall forces among atoms and so it exists as a gas at room temperature. It forms Pπ bonds.