Writing the known oxidation numbers of potassium and oxygen
\(\underset{+2 \times 1}{K_2}\;\;\;\underset{+2x}{Cr_2}\;\;\;\underset{+7(-2)}{O_7}\;\;\; or \;\;\;\underset{+2 }{K_2}\;\;\;\underset{+2x}{Cr_2}\;\;\;\underset{-14}{O_7}\;\;\; \)
Since sum of oxidation numbers = 0, i.e.
2 + 2x - 14 = 0
2x = +14 - 2
x = +6
Hence the oxidation number of chromium in potassium dichromate is +6.
