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

Sulphur has a melting point (when goes solid to liquid) and boiling point (liquid to gas) higher than room temperature (25 degrees Celcius). Oxygen however has a melting and boiling point below room temperature, and thus exists as a gas.

The difference in boiling points is due to differing forces existing between the two. Stronger forces exist within the sulphur atoms (ie. they are holding the sulphur atoms together more) compared to the forces present within oxygen molecules.