How does the formal charge of an atom in a lewis structure differ from the oxidation number of the atom?

n the page discussing the covalent bond, it is shown that the density of electrons in a covalent bond is shared between bothatoms. When drawing Lewis Structures it is sometimes useful to see which structure can be deemed the best. The Formal Charge is a somewhat artificial device that exists in the minds of chemists (not within the molecules, themselves) to help keep track of electrons in their bonding configurations. The Formal Charge is the charge an atom in a molecule or polyatomic ion would have if all of the bonding electrons were divided equally between atoms in the bond. Here are some rules for determining the Formal Charge on each atom in a molecule or polyatomic ion:Electrons within a Lone Pair on an atom are assigned exclusively to that atom.Half of the electrons in each bond around an atom are assigned to that atom.The Formal Charges on all atoms in a molecule must sum to zero; for a polyatomic ion the Formal Charges must sum to the charge on the ion (which may be positive or negative).The Formal Charge is defined by the relationship:Formal Charge = [number of valence electrons in an isolated atom] - [(number of lone pair electrons) + ½ (number of bonding electrons)] HI 

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