Variable valency is shown by

Two elements $X$ and $Y$ have the following electronic configurations: $X = 1s^2, 2s^2 2p^6, 3s^2 3p^6, 4s^2$ and $Y = 1s^2, 2s^2 2p^6, 3s^2 3p^5$. The expected compound formed by the combination of $X$ and $Y$ is:

Which one of the following elements shows both positive and negative oxidation states?

The valency of inert gases is

The oxidation state of $+1$ for phosphorus is found in:

$A$ compound contains three elements $A$,$B$ and $C$. If the oxidation states are $A = +2$,$B = +5$ and $C = -2$,the possible formula of the compound is