Why is oxygen a gas sulphur a solid at while room temperature ?

Explanation

`O_2` molecules are held together by weak van der Waals. forces because of small size and high electronegativity of oxygen. These van der Waals forces of attraction can be easily overcome by collisions of the molecules at room temperature, therefore, `O_2` exists as a gas at room temperature. In contrast, sulphur shows catenation and forms stronger S-S bonds.
Due to catenation, sulphur forms octa-atomic `S_8` molecules having eight membered puckered ring structure. Because of its bigger size, the forces of attraction holding `S_8` molecules much stronger which cannot be overcome by collision of molecules at room temperature. Hence, sulphur is a solid at room temperature.