formal charge
When you can draw a molecule more than one way, how do you decide which Lewis structure is most reasonable? One handy tie-breaker is the formal charge: a simple bookkeeping number that asks, if we shared every bonding pair perfectly equally, how many electrons would each atom appear to own compared with the free atom?
The recipe is: formal charge equals the number of valence electrons in the free atom, minus the number of lone-pair electrons on that atom, minus half the bonding electrons around it (because the atom 'owns' half of each shared pair). Add up the formal charges and you must recover the overall charge of the molecule or ion. The best Lewis structures usually have formal charges as close to zero as possible, and any negative formal charge sitting on the most electronegative atom. So for two candidate structures of the same species, the one with smaller, better-placed formal charges is the more plausible.
Formal charge is a genuinely useful tool — it helps you rank resonance structures, decide which atom belongs in the centre, and sanity-check a tricky inorganic species — but it is crucial to be honest about what it is not. Formal charge is not the real charge on an atom; it deliberately ignores electronegativity by splitting every bond down the middle. The oxidation state, by contrast, hands every shared pair entirely to the more electronegative atom. Both are bookkeeping formalisms, and the truth lies somewhere between them, with the actual charge measured as a partial charge.
For CO drawn with a triple bond, carbon has one lone pair and three bonds: formal charge = 4 - 2 - 3 = -1; oxygen has one lone pair and three bonds: 6 - 2 - 3 = +1. The minus on carbon, plus on oxygen, looks odd, but it correctly predicts that carbon is the more nucleophilic end — which is why CO binds metals through carbon.
Formal charges on carbon monoxide hint at why it bonds metals carbon-first.
Formal charge is not the actual charge on an atom; it assumes every bond is split exactly in half, ignoring electronegativity. Treat it as a comparison tool between structures, not a physical quantity.