Question

At room temperature and under normal conditions, oxygen is a ___ while the other chalcogens are ___.

Answer 1

Oxygen is the second most electronegative element and also has a small atomic radius as compared to the other chalcogens. These factors combine together to make oxygen stand apart from the other chalcogens in several ways.
Particularly, as we go down the group, the E-E (element – element) single bond becomes stronger while the E-E multiple bonds become weaker. Why?
The fact that Sulphur can exist in a large number of allotropic forms can be explained by the ability of S atoms to catenate owing to the high S–S bond energy of $265kJmo{l}^{-1}$. The O–O bond enthalpy is $146kJmo{l}^{-1}$ thus making the peroxides powerful oxidizing agents. The stability of the S–S bond is the reason why Sulphur catenates and exists as a molecular solid at room temperature. The dioxygen molecule has a $p\pi -p\pi$ double bond and is a non-polar molecule. Hence the van der Waals forces between the molecules are weaker and thus dioxygen exists as a gas at room temperature.

Further, as we go down the group, both the mass and the size of the atoms increase. With the increase of mass, the extent of van der Waals forces also increase. This is the reason why except dioxygen, the other elements all are solids at room temperature.