Formal Charges

[Jessica Tran_3K]

How do you calculate formal charges? I know the formula, but I don't really understand how to apply it. Thank you!

[Kaitlyn Ang 1J]

It's used as a tool to draw Lewis Structures in their most stable form. To do this, you manipulate L and S until FC = 0 (as best as possible).

[rabiasumar2E]

Say you want to find the formal charge of S in SO4^-2. You have to draw the lewis structure and from there you'll find the the numbers to plug into the formal charge formula. Number of valence electrons for S is 6. 'L' represents any lone pairs it may have and in this case it has 0. 'S' represents shared electrons and in this case it's sharing 8 electrons (one bond=two electrons and it has four bonds). Plug all those numbers into the formula given and you should find that the FC is +2. Hope this helps.

[nehashetty_2G]

Also remember that if you are calculating the total formal charge of an ionic compound by adding all the individual formal charges of the elements, you want the final value to equal to the charge of the ionic compound. For example in SO4 2- the final value should add to -2 NOT ZERO. Instead, to determine the best structure, look for the combination that adds up to -2 using as many formal charges equal to 0.

[Bradley Whitworth 4B]

The expression shown in class can be simplified by distributing the negative sign which helps for speed in finding formal charges. So, all it really is is taking the valence electrons of the ion you are looking at, minus the amount of electrons in lone pairs, minus the amount of electrons being shared over 2. This looks like:
FC (formal charge)= V (valence electrons) - L (lone pair electrons) - S/2 (shared electrons/2).