Re: structure of carbon monoxide and how it balances charges

I am going to answer this question assuming that you know about electron 
sharing in covalent bonds, and about valence electrons, and the fact that 
you need 8 valence electrons for a stable filled shell (for most elements; 
only 2 for hydrogen). If you don't, you really need your teacher or a 
textbook.

So in Cl2, each Cl atom has 7 valence electrons. But if one electron from 
each Cl atom is shared with the other, so that there are two shared 
electrons, then each atom can claim a filled valence shell -- 6 of its own 
electrons plus two shared ones -- the bonding electrons!

When we come to a molecule like N2, each nitrogen atom has only 5 valence 
electrons. But by each donating 3 electrons to sharing, we can again get a 
filled valence shell: each nitrogen atom has two of its own valence 
electrons and a share of the six bonding electrons. N2 has a "triple bond".

If you have different elements, like in HF, they can still share electrons. 
The H and F atoms each contribute one valence electron to the bonding; that 
leaves F with 6 electrons of its own and 2 shared ones to make the 8, while 
hydrogen has none of its own electrons, but gets a share of the 2 bonding 
electrons to give it the 2 it needs.

But there is something else happens in this situation. F is much greedier 
than H for electrons, and it grabs the lion's share. Electronegativity is a 
measure of the unequal sharing of electrons -- how strongly an atom holds 
the electrons it has and/or seeks a share of others. It works like this:
F > O > N,Cl,Br > other non metals (e.g. S,C,H) > semimetals (Si,B) > 
metals (Al, Fe, Cu, Pb) > very reactive metals (Ca, Na, K).

With carbon monoxide, something a little peculiar happens. Oxygen has 6 
valence electrons, while carbon only has 4. But the oxygen atom indulges a 
little false generosity. As with N2, there are 10 valence electrons 
altogether. In CO, carbon contributes 2 electrons to the bonding, while 
oxygen contributes 4. That way we finish up with 6 shared electrons -- a 
triple bond. The oxygen atom has 2 of its own valence electrons left, and a 
share of the 6 bonding electrons, to make 8; The carbon atom also has 2 of 
its own valence electrons and a share of the 6 bonding electrons. Because 
the oxygen is more electronegative than the carbon, it will grab the lion's 
share of the 6 shared electrons. In practice it gets close to a 2/3 share. 
Seeing that it contributed 4 of the 6 shared electrons in the first place, 
the whole arrangement is a neutral one. Carbon monoxide has a triple bond. 
It is almost completely non-polar. It has the same electron arrangement as 
N2, and in fact its physical properties and many (but by no means all) of 
its chemical properties are almost identical to those of N2.

Finally a word of warning: All of this description of bonding is in terms 
of a very convenient and successful simple model that chemists use in 
describing bonding. A deep understanding of bonding is hidden in some 
enormously complicated quantum mechanics.