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Determination
of Formal Charge
- Unshared pairs
belong entirely to the atom on which they reside.
- Shared pairs
of electrons are divided evenly between the atoms by which they
are shared: half "belong" to one atom, and half "belong"
to the other. (This way of counting electrons is different than that
used for counting octets. When you count for an octet, shared pairs
are counted twice, once for each of the sharing atoms.)
2. Compare the number
of valence electrons that "belong" to each atom to the number
of valence electrons brought into bonding by that atom.
- Carbon always brings
4 valence electrons into bonding.
- Nitrogen always
brings 5 valence electrons into bonding.
- Oxygen always brings
6 valence electrons into bonding.
- The halogens always
bring 7 valence electrons into bonding.
- Hydrogen always
brings 1 valence electron into bonding.
3. If the number of
valence electrons brought into bonding is different from the number that
"belong" to the atom, the atom has a formal charge.
- If the number of
electrons that "belong" is one more than the number
brought into bonding, the formal charge is -1.
- If the number of
electrons that "belong" is one less than the number
brought into bonding, the formal charge is +1.
- etc.
- If the number of
electrons that "belong" is the same as the number brought
into bonding, the formal charge is 0.
4. The sum of the
formal charges is the charge of the species.
- If the sum of the
formal charges is 0, the species is a molecule.
- If the sum of the
formal charges is not 0, the species is an ion.
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