Drawing Lewis Structures

Demonstration of process:

We will draw the Lewis structure for the carbonate ion, CO32-

Step 1: Look up the group numbers for all of the atoms in the formula to determine the number of valence electrons on each atom.
Examples: In carbonate CO32-,
carbon is in group 4A; so it has four valence electrons, 4 e-.
oxygen is in group 6A; so it has six valence electrons, 6 e-

Step 2: Calculate the total number of valence electrons contributed by all the atoms.

This requires multiplying the valence electron count for an atom by the number of atoms of the element and then adding the valence electron numbers for all atoms in the formula.

Example: Carbon dioxide, CO32-
Total electrons = one carbon (4 e-) + three oxygen (3 x 6 e-) = 22 e-; twenty two valence electrons total

Step 3: Check to see if there is a charge on the formula unit. Adjust the total in Step 2 by the number of electrons added or removed because of any electrical charge. This gives you the final total of valence electrons that are available to hold the particle together.
Carbonate ion, CO32- has a charge of 2- which means there are 2 additional electrons.

total of 4 (carbon)+(3x6)(oxygen)+2(negative charge)=24 valence electrons.

Step 4: Identify central atom/atoms in the formula unit.
The central atom is carbon because it forms the most bonds and has the lower electronegativity.

 

Step 5: Think of your structure as a target. Write down the symbol for the central atom in the "bullseye". Then arrange the symbols for the oxygen atoms evenly in the next ring around the center atom.
For example, in carbonate acid, write down the C for the central atom; then put the 3 O symbols symmetrically around the C.

Step 6: Divide the total number of valence electrons for the formula unit (Step 3) by 2 so you know the number of "electron pairs" for the particle.
These electron pairs are used to make the bonds between atoms and to complete the octets in the valence shells of all atoms, except hydrogen, in the formula unit; each hydrogen atom needs only a "duet" or only 2 electrons in its valence shell.

For example, in carbonate ion, Steps 1, 2, and 3 would tell you that the molecule contains 24 valence electrons. Therefore its bond, octets, and duets are done with 12 pairs of electrons.

Step 7: Connect the center atom with a single bond (1 pair of electrons) to each neighboring atom in the second ring. Then, using a single bond, connect each atom in the second ring to its neighbor in the third ring. Count the number of electron pairs used.
For example, in carbonate ion you would connect the C to each of the O's. This would use a total of 3 electron pairs.

Step 8: Subtract the number of electron pairs used in Step 7 from the total number of electron pairs for the particle (Step 6). Use these remaining electron pairs to fill out the octets for atoms in the second ring.
For example, in carbonate ion you used 3 pairs of electrons in Step 7, leaving
12 - 3 = 9 pairs available to finish the octets.

Step 9: The almost last step is verification that the atoms each have an octet or duet. If they do you are finished.

If the central atom lacks an octet when only single bonds are used you need to move a lone electron pair to make one or more multiple bonds. This may mean double or triple bonds as appropriate. NOTE: Any lone pair can be used here.

The negative two charge is spread over the entire ion.