The oxidation state of $Cr$ in $Cr_2O_7^{2-}$ is

Oxidation number of $N$ in ammonium nitrate is

Assign the oxidation number to the underlined element in each of the following species:
$1$. $K_2\underline{Mn}O_4$
$2$. $Ca\underline{O}_2$
$3$. $Na\underline{B}H_4$
$4$. $KAl(\underline{S}O_4)_2 \cdot 12H_2O$

Which one of the following has the highest oxidation number of iodine?

From the given list,the number of compounds with $+4$ oxidation state of Sulphur is $ . . . . . . $.
$SO_3, H_2SO_3, SOCl_2, SF_4, BaSO_4, H_2S_2O_7$

Suggest a list of substances where carbon can exhibit oxidation states from $-4$ to $+4$ and nitrogen can exhibit oxidation states from $-3$ to $+5$.