How To Calculate The Formal Charge of an Atom:
Formal Charge = Valence Electrons - [Bonds + Dots]
(1) Determine the number of valence electrons of the atom using the periodic table.
(2) Count the number of bonds attached to the atom. A double bond has a value of 2 and a triple bond has a value of 3.
(3) Count the number of dots. Each lone pair counts as 2 dots.
(4) Plug in the values into the formula above.
The formal charge of an atom is different from the oxidation state of the atom. The formal charge calculation does not take into account the electronegativity of the atom but assumes all bonding electrons are shared equally. In contrast, the oxidation number of an atom depends on the relative electronegativity of the atom in the molecule.
A good example of this is H2O. The formal charge of the Oxygen atom is zero.
Formal Charge = 6 Valence Electrons - (2 Bonds + 4 Dots) = 0
The oxidation number of Oxygen is -2 in H2O which is different than the formal charge of zero.
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