Formal+Charge

__What is Formal Charge?__
Formal Charge is the charge that an atom would have if all atoms had the same electronegativity. This would be if all bonding electon pairs would be shared equally between the two atoms.

__Calculating Formal Charge__
There is a formula to find the formal charge of any atom in a Lewis structure. This formula is:


 * Formal Charge= # of valence electrons - (# of unbonded electrons+(1/2 bonded electrons))**

Example:

CN- ---> [ : C ≡ N : ] -
Carbon has 4 valence electrons, 2 unbonded electrons, 3 bonded electrons

4 - ( 2 + 3 ) = -1

So Carbon has a -1 charge

__Alternative Lewis Structures__
Formal charges can help determine which Lewis Structure works best. In a SCN- molecule there are three different ways to draw a Lewis structure, but which one works best? From the picture below we find that the first one works best.



Sources: Chemistry The Central Science Ninth Edition http://employees.oneonta.edu/kotzjc/POWERPOINT/Formal_charge.pdf