Drawing Lewis Diagrams
1. Write the skeletal structure (the arrangement of atoms within the molecule):
Carbon forms 4 bonds (a double bond is 2 bonds) and there are often C-C bonds
in the "center" of the molecule.
Hydrogen forms only 1 bond and is therefore a terminal atom (on the outside of
the molecule).
Oxygen usually forms 2 bonds.
Nitrogen usually forms 3 bonds (may form 4 in a cation like NH4+).
2. Count the total number of valence electrons:
Group number for each element = # valence electrons
Add electrons for negatively charged ions.
Subtract electrons for positively charged ions.
3. Join atoms by single bonds.
Single bond = 1 pair of electrons.
4. Place lone pairs on terminal atoms (except H) to complete their octets
Octet = 4 electron pairs around an atom = 8 electrons
Hydrogen can have only 2 electrons.
5. If there are more electrons left, place them as lone pairs on remaining atoms to
complete their octets.
6. If there are still atoms with incomplete octets, use the electrons from surrounding
atoms to make double or triple bonds.
Do not add electrons. "Borrow" them from surrounding atoms.
Double bond = 2 pairs of electrons
Triple bond = 3 pairs of electrons
Only C, N, O, P and S form multiple bonds.
F and Cl do not form multiple bonds.
Examples:
CH2Cl2
CO2
BF3
CH3CONH2