Here is the touchy-feely method: Every atom has a core or kernel charge - it is the same as the number of valence electrons (the group number ignoring the 10's digit). It can also help one better identify which atoms might be more positive or more negative in the structure. +2, –2, +3, –3 are right out! Here's you a graphic to help.įirst and foremost, know that formal charge is not a real charge at all - it is an electron counting inventory system that helps us decide on best structures. Stick to the sweet spot of formal charges. Do NOT ever consider a structure as valid or correct when any atom has a formal charge outside of the –1, 0, +1 range. Now don't get carried away on drifting from zero. These are not only "ok" but also necessary on certain structures. This is when we allow both +1 and –1 as formal charges also. Unfortunately, sometimes it is just impossible to get all zeroes for formal charge on a structure. And, since zero is the winner of smallest, we say the best structure is the one with the most zeros in it. The right/correct structure is always the one with the smallest absolute values of formal charge. Often, one structure is the preferred and "right" structure. But sometimes you can come up with two or more structures that both satisfy the octet rule and you're faced with "which one?". Following the octet rule is very helpful and should be your first approach. Formal charge is a construct that helps us decide on the "best" structure when making attempts.
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