Question

What is the oxidation state of zinc in `"ZnO"_2`?

Answer 1

This is zinc peroxide (not "zinc(IV)" oxide!), i.e. it contains `2 xx "O"` in the `color(red)(-1)` oxidation state.

The way you would know that is by already knowing that zinc has a maximum oxidation state of `+2`. Its electron configuration is `[Ar] 3d^10 4s^2`, so losing more than two electrons is nearly impossible (it would require freeing up a quantum state in the `3d` outer-core orbitals).

Hence, the zinc atom has an oxidation state of `color(blue)(+2)` (not `+4`, and not `+1`).

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You may accidentally say that this is simply `2 xx "O"^(2-)` for the anion(s). You may also miss that for the anion without prior knowledge or without looking it up, it is a non-innocent ligand that could either be `"O"_2^(-)` (the superoxide anion) or `"O"_2^(2-)` (the peroxide anion).

Either misconception could lead you to accidentally say that zinc has an oxidation state of `+4` or `+1`. Neither is correct!