Assign and sum of the oxidation number various atoms to Cr in ${{K}{2}}$C${{r}{2}}$${{O}{7}}$.
Let the oxidation number of Cr be x
Sum of the oxidation numbers of various atoms in ${{K}{2}}$C${{r}{2}}$${{O}{7}}$
= 2 x (+1) + 2x + 7 x (-2)
= 2 + 2x-14 = 2x-12
x = + 6
.•. Oxidation number of Cr in ${{K}{2}}$C${{r}{2}}$${{O}{7}}$ = +6