Formal charge is the term for the electrical charge on an atom.
The charge on an atom depends on the balance between the negative charges of electrons and the positive charges of protons. Since only the valence electrons can be gained or lost, the following equation can be used to calculate formal charges.
formal charge = "# of valence electrons" - 1/2 (# of shared electrons) - # of non-bonding electrons
The "number of valence electrons" will be the Roman numeral directly above that element's column in the periodic table...the group number.
Remember that if a pair of electrons is shared in a chemical bond, then neither atom really owns both electrons unless the bond is an ionic bond. That is why the equation multiplies the number of shared electrons by one half.