Question

Which one of the following molecules exists as tetra – atomic molecule?
a.Sulphur
b. Helium
c. Oxygen
d. Phosphorus

Answer 1

A molecule is a substance having a definite number of atoms. A molecule that has one atom is called mono – atomic, a molecule with two atoms is called di – atomic, a molecule with three atoms is called tri – atomic, while with four atoms a molecule is called tetra – atomic. The elementary forms of the higher group molecules exist in more than one atomic form.

Complete answer:
We have been given sulfur, helium, oxygen and phosphorus molecules. Among them we have to identify the molecule which is tetra – atomic. A tetra – atomic molecule contains 4 atoms of that element or different element.
Among the given elements, sulphur exist as ${{S}_{8}}$, helium exist as He, oxygen exist as ${{O}_{2}}$, while phosphorus exist as ${{P}_{4}}$. Thus phosphorus has tetra – atomic arrangement.
Phosphorus in its elementary form is present in a tetrahedral molecule as ${{P}_{4}}$. Each phosphorus atom in this molecule is bonded to three other phosphorus atoms by saturated single bonds. As the phosphorus is from the group 15, it has partially filled p – orbital, but unlike nitrogen it cannot form triple bonds due to its large size, therefore it has a tetra – atomic arrangement where the phosphorus atoms are bonded by sigma bonds. The structure of ${{P}_{4}}$ is:

Hence, the molecule that exists as tetra – atomic molecule is phosphorus, so option d is correct.

Note:
Phosphorus exists in allotropic forms that have different physical compositions. It has allotropes of black, white and red phosphorus. White phosphorus is highly reactive and has the basic tetrahedral structure with angular strain $60{}^\circ$ that makes it reactive, while red phosphorus is less reactive and has a polymeric chain structure having chains of ${{P}_{4}}$ tetrahedral. Black phosphorus is obtained in two forms, $\alpha ,\beta$ obtained by heating red and white phosphorus respectively.