Formal charge
Formal charge is an electron-bookkeeping device that gives the charge on a bonded atom in a Lewis structure relative to a nonbonded atom of the same element.
To calculate formal charge compare the number of electrons "owned" by a bonded atom to the number owned by the corresponding nonbonded atom. Assume in your count that
Thus the formal charge at each of these central atoms is zero. Similarly the charge on F and Cl (Group 17) in the structures is zero as each of these owns seven (six nonbonding and one) electrons. This is equal to the number of valence electrons on a nonbonded halogen atom.
To calculate formal charge compare the number of electrons "owned" by a bonded atom to the number owned by the corresponding nonbonded atom. Assume in your count that
- nonbonding electrons at the atom are "owned" by that atom
- bonding electrons are equally shared. Each bonded atom owns one electron of each bond.
 At Xe in XeF4 4 bonding 4 nonbonding = 8 Nonbonded Xe (Group 18) has 8 valence electrons |  At P in PCl3 3 bonding 2 nonbonding = 5 Nonbonded P (Group 15) has 5 valence electrons |  At B in BF3 3 bonding 0 nonbonding = 3 Nonbonded B (Group 13) has 3 valence electrons |
Thus the formal charge at each of these central atoms is zero. Similarly the charge on F and Cl (Group 17) in the structures is zero as each of these owns seven (six nonbonding and one) electrons. This is equal to the number of valence electrons on a nonbonded halogen atom.