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In many sites I see that they take $\ce{C}$ with valence 4, bur our teacher takes it with valence 2. With what valence should I take $\ce{C}$?

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    $\begingroup$ Carbon is typically viewed as having a valence of 4. Its valence can be satisfied by any combination of bonds and lone pairs of electrons. $\endgroup$ – ron Feb 15 '15 at 17:04
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There is resonance between various Lewis structures as:

$$\ce{:\!\overset{\ominus}{C}#\overset{\oplus}{O}\!: ~<->~ :\!C=O\!:: ~<->~:\!\overset{\oplus}{C}-\overset{\ominus}{O}\!:::}$$

The term "valence" is not used much nowadays, and "oxidation state" is used but has a different meaning.

The IUPAC definition of oxidation state requires the oxygen have a -2 oxidation state except in peroxides. All the oxidation states must add up to 0 for a neutral molecule. Therefore the oxidation state of carbon in CO must be +2.

There is an IUPAC definition of valence:

The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.

This imposes a unique valence upon each element, as explained in this article, in which case valence is always 4 for carbon.

There are other definitions of "valence" such as:

The number of hydrogen atoms that can combine with an element in a binary hydride or twice the number of oxygen atoms combining with an element in its oxide or oxides

(Chemistry of the Elements by Greenwood et al.)

According to this definition, carbon would have a valence of 2 in CO.

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  • $\begingroup$ How many valence electrons are there in these cases? I'm only considering the middle example since I'm not getting the others. In that case C has 4 valence electrons, but you said it has the valence 2. Can you explain me please? $\endgroup$ – user13411 Feb 15 '15 at 16:12
  • $\begingroup$ So it's correct the connection C=O, in which C has no unpaired electrons. $\endgroup$ – user13411 Feb 15 '15 at 17:07
  • $\begingroup$ C has one lone pair of electrons in CO. Do you see the lone pair in each Lewis structure of my answer to the left of C. $\endgroup$ – DavePhD Feb 15 '15 at 17:09
  • $\begingroup$ I do, but if it has a lone pair it means that the number of its electrons is bigger than 2 and we said that it only has 2 valce electrons (the C valence is 2 in this case, as we said) $\endgroup$ – user13411 Feb 15 '15 at 17:13
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    $\begingroup$ "valence of 2" does not mean "has 2 valence electrons". Read the two definitions of valence quote in the answer and see that there is no mention of electrons. $\endgroup$ – DavePhD Feb 15 '15 at 17:17

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