The formal oxidation numbers of $Cr$ and $Cl$ in the ions $Cr_2O_7^{2-}$ and $ClO_3^{-}$ respectively are

The oxidation number of sulphur in $S_2O_3^{2-}$ is:

The oxidation number of $C$ in $HNC$ is:

The oxidation number of sulphur in $S_8$,$S_2F_2$ and $H_2S$ respectively are:

What is the change in oxidation number of selenium in the following redox reaction?
$SeO_{3(aq)}^{2-} + Cl_{2(g)} + 2OH^- \longrightarrow SeO_{4(aq)}^{2-} + 2Cl_{(aq)}^- + H_2O$

Determine the oxidation state of $Cr$ in $K_2Cr_2O_7$.