# Is valency of an element equal to sum of covalency and electrovalence? Why do we calculate valency and oxidation number? [closed]

About valency books say following things:

1- Valency is combining capacity of an atom with number of hydrogen atoms or double of oxygen atoms with which the atom combines.

Doubt A: what is valency of C in C2H2 and CO?

2- Valency is number of electrons gained, lost or shared by atom to complete its octate.

Doubts B: in CO carbon shares 2 electrons and gains 2 electrons. There are triple bond between C and O. Should valency of carbon be 2 or 3 or 4?

Then

3- Electrovalency = number of charge on an ion.

And

4- Covalency = number of shared electrons.

Then books say that

5- For an element covalency and electrovalency are equal.

Then books say that

6- Valency of carbon is 4.

Now when I see carbon monoxide (CO), I am not able to figure out valency of carbon or oxygen.

Then comes oxidation number (hypothetical charge), which seems to be similar to electrovalency (actual charge), but still some online sources say these are different things.

Doubt C: Why do we calculate oxidation number? How is it different from electrovalency? Is the only difference hypothetical and actual?

For me, 1) Valency was a very useful value given by the number of doublets and the number of charges joining two atoms in the Lewis representation. It has been replaced today by the notions of covalency and ionic charges. 2) Electrovalency is an obsolete reality that is not used any more. 3) Oxidation number of an atom is a number that can be calculated using the following set of rules : a) Pure elements O.N. is zero; b) In compounds the sum of all O.N. is zero; c) In ions, the sum of the O.N. is equal to the charge of the ion; d) Fluor O.N. is $$-1$$ as O.N. e) Elements O.N. from the $$1$$st and $$2$$nd columns are the number of the column, f) Oxygen O.N. is $$-2$$ (except in compounds with fluor, in H2O2 and other similar peroxides), g) For other atoms in polyatomic molecules or ions, the O.N. is an unknown $$x$$ that can be obtained by applying the previous rules.