A Sure-Fire Way to Draw Lewis Structures!

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:

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.

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:

This method is basically the same one you learned above, except that there are a few extra rules to keep track of. Changes in the procedure above are outlined in red for your convenience.

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. For polyatomic anions, add the charge of the ion to the number of valence electrons. For polyatomic cations, subtract the charge of the ion from the number of valence electrons.

2) Figure out how many octet electrons the molecule should have, using the octet rule: The octet rule tells us that all atoms (including boron) 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.

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:

Hydrogen and the halogens bond once.
The family oxygen is in bonds one, two, or three times.
The family nitrogen is in bonds two, three, or four times
Boron usually bonds four times.
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.

7) To find the charge on each atom, compare the number of electrons that each atom has to the number of valence electrons it usually has. For this purpose, each bond counts as one electron and each lone pair counts as two electrons. For example, in CO₂ above, carbon has four electrons (because it has four bonds) and oxygen has six (two bonds + 4 lone pair electrons). If the number of electrons that the atom has is more than the normal number of valence electrons, the atom has a negative charge. If the number is less than the normal number of valence electrons, the atom has a positive charge. If it's the same, the atom has no charge at all.