The oxidation number of nitrogen atoms in $NH_4NO_3$ are

Fill in the blanks:
$(1)$ During the reaction $CN^{-} \to CNO^{-}$,......... is added.
$(2)$ The maximum positive oxidation number for any chalcogen compound may be ......... .

Which of the following pairs will have different oxidation numbers for oxygen?

What is the oxidation state of nitrogen in $N_3H$?

What is the oxidation number of $Cr$ in $K_{2}Cr_{2}O_{7}$?

Oxidation number of $P$ in $Mg_2P_2O_7$ is