I know that oxidation number is the charge any species has in its molecular or ion form. And valency is the number of electrons a species contain in its valence shell, that always tends to be used in bond formation. But is there any relation between the oxidation number and those valence electrons?

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    $\begingroup$ Better relearn from the start - in particular oxidation number has little to do with actual charges. $\endgroup$
    – Mithoron
    Jun 12 '21 at 15:13

There is no simple and general relationship between valency and oxidation number. Look at the example of Oxygen. It depends of experimental evidences.

Oxygen has always valency $2$. But it has an oxidation number equal to $-2$ in the vast majority of its compounds, like water $\ce{H2O}$ or $\ce{CO2}$. It can also be at oxidation number $-1$ in $\ce{H2O2}$, or at oxidation number zero in $\ce{O2}$. It can be at $+1$ in the molecule $\ce{O2F2}$, and at $+2$ in $\ce{OF2}$


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