Q.Why is dioxygen a gas but sulphur a solid?

Step 1: Bonding in oxygen.

Oxygen's small atomic size allows effective sideways ($p\pi$–$p\pi$) overlap between the p-orbitals of two oxygen atoms, forming a strong $O=O$ double bond (one $\sigma$ + one $\pi$). Oxygen therefore exists as small, light diatomic molecules, $O_2$.

Step 2: Weak intermolecular forces in $O_2$.

Since $O_2$ molecules are small and non-polar, the only intermolecular attraction between them is weak van der Waals (London dispersion) force. This is too weak to hold the molecules together as a condensed phase at ordinary temperature, so oxygen exists as a gas.

Step 3: Bonding in sulphur.

Sulphur atoms are considerably larger than oxygen atoms, so effective $p\pi$–$p\pi$ overlap between two S atoms is not possible — sulphur cannot form a stable $S=S$ double bond analogous to $O=O$. Instead, sulphur catenates through single S–S ($\sigma$) bonds.

Step 4: Consequence — $S_8$ and solid state. …

Contrast oxygen's ability to form a strong O=O π bond (small diatomic molecule, weak intermolecular forces, gas) with sulphur's inability to do so, leading it to cat …

• Do not just say "sulphur atoms are bigger, so it's a solid" — the mechanism (inability to π-bond, so it catenates into large S₈ molecules) must be explained, as this is the actual reason for the greater intermolecular forces.
• $S_8$ is a ring (crown-shaped), not a chain or a diatomic S₂ analogue — describing sulphur's molecular form incorrectly is a common error. …