Oxygen is a gas, but other elements of group 16 are solids at room temperature. Why? 

Explanation

Oxygen exists as isolated diatomic molecules. Between atoms of oxygen molecule the attractions are strong, but inter molecular attractions are very weak. Hence oxygen is a gas.
Other elements of group 16, have complex states. The tendency to form multiple bonds between identical atoms decreases with an increase in atomic weight. Bonding ability between many atoms of same element increases. Molecular complexity and van der Waals forces increase. Hence, they are solids.