Due to the small size and high electronegativity, oxygen forms \(p \pi-p \pi\) multiple bonds. As a result, oxygen exists as diatomic \(\left(\mathrm{O}_2\right)\) molecules. These molecules are held together by weak vander Waal's forces of attraction which can be overcome by collisions of the molecules at room temperature. Therefore, \(\mathrm{O}_2\) is a gas at room temperature. Due to its bigger size and lower electronegativity, sulphur does not form \(p \pi-p \pi\) multiple bonds. It prefers to form \(\mathrm{S}-\mathrm{S}\) single bonds. \(\mathrm{S}-\mathrm{S}\) single bond is stronger then \(\mathrm{O}-\mathrm{O}\) single bond. Thus, sulphur has higher tendency for catenation than oxygen. Due to higher tendency for catenation and lower tendency for \(p \pi-p \pi\) multiple bonds sulphur exits as octa-atomic \(\left(\mathrm{S}_8\right)\) molecule. Due to bigger size, the force of attraction holding the \(\mathrm{S}_8\) molecules together are much stronger which cannot be overcome by collisions of molecules at room temperature. Therefore, sulphur is solid at room temperature.