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Isn't oxidation number a weird concept? It assumes complete transfer of an electron from a more EN to a less EN atom per covalent bond formed. This assumption isn't even true. We know that partial $δ^+$ and $δ^-$ charges develop on the atom. Why did we introduce such a weird concept? And what is it good for?

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    $\begingroup$ If assuming something weird helps us predict something better, we assume it. For example we also assume hybridization which in reality doesn't happen. But it helps us predict structures. Oxidation state likewise helps us predicting the oxidizing or reducing properties of a substance. $\endgroup$ Apr 16, 2021 at 3:45
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    $\begingroup$ In short: if something looks weird, but works, then it isn't all that weird. $\endgroup$ Apr 16, 2021 at 8:58
  • $\begingroup$ " It assumes complete transfer of an electron" - it doesn't assume anything. $\endgroup$
    – Mithoron
    Apr 16, 2021 at 23:05
  • $\begingroup$ chemistry.stackexchange.com/questions/69798/… $\endgroup$
    – Mithoron
    Apr 16, 2021 at 23:08

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Oxidation number is a deliberate approximation to make keeping track of electrons in molecules easy

You are quite right to say that oxidation number isn't "true". It is clearly a simplified idea and not a perfect representation of "reality" whatever that is.

But there is a point to it. That point is to enable chemists to keep track of the electron count in molecules. Electron distributions may be infinitely variable in bonds but electrons are discrete and keeping count of them is important. In transition metal complexes, for example, it is important to make sure all the bonding electrons are correctly accounted for. Many transition metals can have variable oxidation states with very different properties and the (crude) approximation of where those electrons are helps us classify those differences even when the metal-ligand bonds are covalent and the electron distributions are not clear-cut or ionic.

So there is no intent to correctly represent the distribution of charge in bonds. But pretending the oxidation number is an integer helps keep track of things and is often very useful at explaining properties.

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    $\begingroup$ Another example is from biochemistry: It would be very difficult to count all valence electrons in a protein just to figure out what is going on when a disulfide forms. Instead, we say that the oxidation number of the sulfur atom in cysteine changes, and all other oxidation numbers are untouched. That tells us that there has to be an oxidizing agent that gets reduced to accept the electrons released during the disulfide formation. $\endgroup$
    – Karsten
    Apr 16, 2021 at 11:46

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