Question

Give reason for the following:
Oxygen is a gas but sulphur is a solid.

Answer 1

Oxygen and sulphur both are the elements of group 16 in the periodic table. But oxygen is present at the second period and sulphur is present in the 3rd Period of the periodic table. All the second period elements show some exceptional behavior from the rest of the group, because these 2nd group elements are small in size. And they don’t have d-orbitals.

Complete answer:
In gaseous molecules the intermolecular forces are weak and hence the distance between each molecule is large in gaseous state, while in case of solid the intermolecular forces are strong and hence the distance between each molecule is very small, hence they have definite structure while gases have no definite structure.
Here oxygen is small in size as compared to the sulphur molecule, so oxygen will have high electronegativity. Also oxygen does not have d-orbital in it, they have only p- orbital. Since the size is small, after forming a single bond $O - O$ , the pi- electron clouds are very close and hence can easily form a double bond. Hence dioxygen exists as $O = O$. Hence the dioxygen exists as a discrete molecule and the intermolecular forces between the oxygen molecules that are Van der Waals forces are weak. Therefore dioxygen exists as gas.
On the other hand sulphur is larger in size than oxygen, and also has less electronegativity and hence the pi orbitals in this case are not close enough to make pi bonds and hence prefer to form single bonds. And exits as a polymeric structure ${S_8}$. 8 molecules of sulphur induce more dipole and create strong force between the molecules. Or we can say the Van der Waals forces are strong and hence sulphur exists in solid form.

Note:
There is a large difference between the melting and boiling point of oxygen and sulphur, and it can be explained on the basis of atomicity. As oxygen exists as a diatomic molecule in nature and sulphur exists as a polyatomic molecule. And the reason for different atomicity is explained above.