Why is the valance of carbonate CO3 (-2)

Question

carbon has electronic configuration 1s2 2s2 2p2. the valency of carbon is 4. then why not its valency is -1

Answer 1

By the octet rule, all atoms in this case should end up with 8 valence electrons for the most stable configuration. We have three atoms, so you can think about it as our molecule needing 8*4 = 32 valence electrons. So, think about it like this:

Carbon has 4 valence electrons.

Each oxygen has 6 valence electrons.

We have a total of 22 valence electrons.

Carbon can have at most four bonds.

If we use all four bonds (which is possible, with one double bond and two single bonds to oxygen - although this isn't really what's happening as there will be resonance), then we will have 8 shared electrons and 14 free electrons. Each shared electron can be counted as two electrons since they act as a valence electron on two atoms, and therefore we have

14+8*2 = 14+16 = 30 valence electrons.

Obviously, this doesn't fulfill our 32 valence electron requirement. Thus, two extra electrons had to have come from somewhere else to stabilize this molecule. These two electrons lead to the -2 formal charge.

Hope this helps!

Answer 2

It's all basic math.

The carbonate ion is known to have an oxidation number of -2.  For instance consider a compound like sodium carbonate, Na2CO3.  Sodium has an oxidation number of +1.  Since the sum of the oxidation numbers of a compound must be zero, and the sum of the oxidation numbers is equal to the charge on an ion.then carbonate is+

CO3^2- can written as:

+x -2 -2 -2 = -2

[ C O O O ]2-

Solve for x

x = +4