Chemistry-Formal Charge

I have a test tomorrow and I don't really understand how to do formal charge and I've been looking at it and it just isn't clicking. Thank you for your help.

Count the electrons around the atom. If the electron(s) is/are shared with another atom, half goes to one atom and half to the other. Subtract the number the atom has (with the charge) from what it should have (with the charge) and that is the formal charge. These things are hard to do on the Internet but here is an example.
:N:::N:O: plus .. above O and .. below O. Count them and you should have 16 electrons.
For the left N, the first two are not shared so that counts as two. the next six are shared so that counts as three (1/2 of 6). That gives the first N five electrons. It had 5 to start with so the FC is 0 [-5-(-5)=0]. The second N has three on the left (again, 1/2 of 6) and a shared pair on the right (which counts as one electron). That adds to 4, it had 5 to begin with so the FC is +1 [-4 -(-5)=+1]. For oxygen, there are six unshared and one pair shared (which counts as one) and O has six to start with so we have -7 -(-6)=-1
Please explain where you have trouble if you are still confused.

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asked by Katy

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