The decomposition of ammonium dichromate
$(NH_4)_2Cr_2O_7$$Cr_2O_3$ $+ $ $N_2$ $ + $ $4H_2O$
formes chromium(III) oxide
(green solid observed after reaction),
nitrogen,
(colourless gas)
and water.
Find atoms which change oxidation number
$Cr$,
OS(+6) in $Cr_2O_7^{2-}$ $\rightarrow$ OS(+3) in $Cr_2O_3$
$N$,
OS(-3) in $NH_4^+$ $\rightarrow$ OS(0) in $N_2$
$Cr_2O_3$ $+ $ $N_2$ $ + $ $4H_2O$
formes chromium(III) oxide
(green solid observed after reaction),
nitrogen,
(colourless gas)
and water.
Find atoms which change oxidation number 
$Cr$,
OS(+6) in $Cr_2O_7^{2-}$ $\rightarrow$ OS(+3) in $Cr_2O_3$
$N$,
OS(-3) in $NH_4^+$ $\rightarrow$ OS(0) in $N_2$