STEPS FOR DRAWING LEWIS STRUCTURES
1. Given the molecular formula, add up the valence electrons in the atoms.
Example: C2H4O 2(4) + 4(1) + 1(6) = 1
Note: In the case of an ion if it has a โ+โ charge subtract that number from the total valence
electrons. If it has a โ-โ charge, add that number to the total valence electrons. Example: NH4+
has 8 valence electrons (5 + 4 โ 1) and NO3- has 24 valence electrons (5 + 18 + 1).
2. Arrange the atoms with one or two central atoms and the others around it.
a. H atoms are always on the outside because they always only have 1 bond.
b. O atoms are usually on the outside (not central) unless combined with H like H2O
c. C atoms are always a central atom
H H
H C C O H
3. Connect the atoms with lines (bonds) to the central atom.
H H
H C C O H
4. Subtract the bonded electrons (2 per line) from the total valence electrons: 18 โ 6(2) = 6
5. Place these extra electrons around outside atoms until they have a complete octet, then put the
remainder on inside atoms. In this case, put the 4 electrons around the oxygen atom and two
electrons on one carbon atom. Always place electrons around atoms in pairs.
H H
H C C O H
6. Count electrons around each atom to be sure there is a complete octet, except for H which only has a
duet. One of the carbon atoms does not have a complete octet.
7. If one or more of the central atoms do not have a complete octet then use some of the non-bonding
electron pairs to make a bonding pair (i.e., make multiple bonds) so that that all atoms end up with
complete octets.
H H
H C C O H
H H
H C C O H
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