How to draw Lewis structures for molecules that contain no
charged atoms
1) Count the total valence electrons for the molecule: To do this, find the
number of valence electrons for each atom in the molecule, and add them up.
2) Figure out how many octet electrons the molecule should have, using
the octet rule: The octet rule tells us that all atoms want eight valence electrons
(except for hydrogen, which wants only two), so they can be like the nearest
noble gas. Use the octet rule to figure out how many electrons each atom in the
molecule should have, and add them up. The only weird element is boron - it
wants six electrons.
3) Subtract the valence electrons from octet electrons: Or, in other words,
subtract the number you found in #1 above from the number you found in #2
above. The answer you get will be equal to the number of bonding electrons in
the molecule.
4) Divide the number of bonding electrons by two: Remember, because
every bond has two electrons, the number of bonds in the molecule will be equal
to the number of bonding electrons divided by two.
5) Draw an arrangement of the atoms for the molecule that contains the
number of bonds you found in #4 above: Some handy rules to remember are
these:
o
o
o
o
Hydrogen and the halogens bond once.
The family oxygen is in bonds twice.
The family nitrogen is in bonds three times. So does boron.
The family carbon is in bonds four times.
A good thing to do is to bond all the atoms together by single bonds, and then
add the multiple bonds until the rules above are followed.
6) Find the number of lone pair (nonbonding) electrons by subtracting the
bonding electrons (#3 above) from the valence electrons (#1 above).
Arrange these around the atoms until all of them satisfy the octet rule:
Remember, ALL elements EXCEPT hydrogen want eight electrons around them,
total. Hydrogen only wants two electrons.
Let's do an example: CO2
Note: Each of the numbers below correspond to the same numbered step above.
1) The number of valence electrons is 16. (Carbon has four electrons, and each
of the oxygens have six, for a total of 4 + 12 = 16 electrons).
2) The number of octet electrons is equal to 24. (Carbon wants eight electrons,
and each of the oxygens want eight electrons, for a total of 8+16 = 24 electrons).
3) The number of bonding electrons is equal to the octet electrons minus the
valence electrons, or 8.
4) The number of bonds is equal to the number of bonding electrons divided by
two, because there are two electrons per bond. As a result, in CO2, the number
of bonds is equal to 4. (Because 8/2 is 4).
5) If we arrange the molecule so that the atoms are held together by four bonds,
we find that the only way to do it so that we get the following pattern: O=C=O,
where carbon is double-bonded to both oxygen atoms.
6) The number of nonbonding electrons is equal to the number of valence
electrons (from #1) minus the number of bonding electrons (from #3), which in
our case equals 16 - 8, or 8. Looking at our structure, we see that carbon already
has eight electrons around it. Each oxygen, though, only has four electrons
around it. To complete the picture, each oxygen needs to have two sets of
nonbonding electrons, as in this Lewis structure: