Rules for Writing Lewis Dot Structures:
1. Write the bonding sequence:
a) For simple molecules and polyatomic ions, the first atom in
the formula is usually the central atom and all other atoms
are bonded to it.
b) Hydrogen is always a terminal atom.
c) In general, molecules with symmetrical formulas tend to
have symmetrical structures. For example S2Br2 has the
following skeleton structure: Br-S-S-Br. Note that the less
negative sulfur atoms follow rule 1a.
d) The oxy-acids are an exception to rule 1a. The oxygen
atoms of oxyacids are bonded to the central atom and the
hydrogen atoms are bonded to the oxygen atoms. For
example, chlorosulfonic acid (HSO3Cl) has the following
skeleton:
O
S
Cl
OO H
e) When several possible skeletons exist, choose the one in
which the atoms have their usual number of covalent
bonds.
2. Add up the total number of valence electrons in the molecule.
Do this by adding together the valence electrons for each
atom. Do not forget to add or subtract electrons for anions or
cations.
3. Subtract from this total the number of electrons used to form
single bonds in rule 1. Remember each single bond counts as
two electrons.
4. Complete octets of all atoms using nonbonding electron pairs.
Subtract this number of electrons from those not used to form
the single bonds.
.
5. If there are not enough electrons for rule 4, consider multiple
bonds. (C, N, O, and S often form multiple bonds.) You will
need one multiple bond for every two electrons you are short
in rule 4.
6. If there are too many electrons for rule 4, assign the extra
electrons to the central atom.
