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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?

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    $\begingroup$ There are Wikipedia pages that will answer most of your questions: en.wikipedia.org/wiki/Valence_(chemistry), en.wikipedia.org/wiki/Covalent_bond and en.wikipedia.org/wiki/Ionic_bonding $\endgroup$
    – Buck Thorn ♦
    Jan 19 at 10:48
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    $\begingroup$ Carbon monoxide is admittedly a very strange molecule: en.wikipedia.org/wiki/Carbon_monoxide The usefulness of the concept of valency has its limitations... $\endgroup$
    – Buck Thorn ♦
    Jan 19 at 10:49
  • $\begingroup$ I needed to know how to find valency from periodic table so that I can write correct formula of compounds. I have been reading links including similar questions on this website as well and I think that idea of valency to write chemical formula is much limited. I still don't understand why C in CO has valency 2. Is valency here implies covalency? What about 2 electrons borrowed from Oxygen? Why would oxygen give up 2 electrons when it is more electronegative than carbon? $\endgroup$
    – Level1
    Jan 19 at 17:09
  • $\begingroup$ The concepts of valency and Lewis structures are convenient but imperfect tools, and CO is tricky. Some explanations of bonding in CO involve resonance between various structures. This can be used to accommodate observations such as a bond order between 2 and 3 and a partial negative charge on C, but it is difficult to explain before the fact. CO valency would therefore lie between 2 and 3. Regarding selecting Lewis structures, Pauling suggest a rule of thumb called "the electroneutrality principle" which suggests that preferred structures keep the partial charges on the atoms to a minimum. $\endgroup$
    – Buck Thorn ♦
    Jan 19 at 18:57
  • $\begingroup$ It is useful to think of atoms as separate characters being swapped and forming new associations. But it is truer to think that atoms and molecules are associations of electrons and nuclei. There are QM rules that ultimately decide where the electrons are mostly found around the nuclei. See for instance: chemistry.stackexchange.com/questions/32161/… $\endgroup$
    – Buck Thorn ♦
    Jan 19 at 19:04

1 Answer 1

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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.

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  • $\begingroup$ Why do we calculate valency if it can't help in writing formula for compounds? Why do we calculate oxidation number? $\endgroup$
    – Level1
    Jan 19 at 16:17
  • $\begingroup$ In textbooks valency due to ions is referred to as electrovalency. $\endgroup$
    – Level1
    Jan 19 at 17:15
  • $\begingroup$ Oxidation numbers are necessary for writing redox chemical equations. $\endgroup$
    – Maurice
    Jan 19 at 21:08

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