In the conversion of K₂Cr₂O₇ to K₂CrO₄, the oxidation number of the following changes

Oxidation state of Cr in K₂Cr₂O₇ is:

$$\begin{gathered} 2\left(1\right)+2(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr})+7(-2)=0 \\ 2+2(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr})-14=0 \\ \mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr}=+6 \end{gathered}$$

Oxidation state of Cr in K₂CrO₄ is:

$$\begin{gathered} 2\left(1\right)+\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr}+4(-2)=0 \\ 2+\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr}-8=0 \\ \mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr}=+6 \end{gathered}$$

Oxidation state of K in K₂Cr₂O₇ is:

$$\begin{gathered} 2(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{K})+2\left(6\right)+7(-2)=0 \\ 2(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{K})+12-14=0 \\ \mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{K}=+1 \end{gathered}$$

Oxidation state of K in K₂CrO₄ is:

$$\begin{gathered} 2(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{K})+6+4(-2)=0 \\ 2(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{K})+6-8=0 \\ \mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{K}=+1 \end{gathered}$$