$$a \mathrm{MnO}_4^{-}+b \mathrm{~S}_2 \mathrm{O}_3^{2-}+\mathrm{H}_2 \mathrm{O} \longrightarrow x \mathrm{MnO}_2 +y \mathrm{SO}_4^{2-}+z \mathrm{OH}^{-}$$

$$a$$ and $$y$$ respectively are

In which of the following compounds, an element exhibits two different oxidation states?

All Cu(II) halides are known, except the iodide, the reason for it is that

Which of the following is not true for oxidation?