7.3: Background - Lewis Structures

Atomic Lewis Structures

To draw an atomic Lewis structure, use the following steps. The carbon atom will be shown as a demonstration.

1. Write the chemical symbol for the element from the periodic table. For carbon:

2. Determine the number of valence electrons based on the periodic table. For carbon, as it is group 4A, this number is 4. If looking at the 1 through 18 column numbers, for groups 1 and 2, the valence electrons match the group number. For groups 13-18, the number of valence electrons is the group number minus 10. For carbon in group 14, 14-10=4 valence electrons.
3. Place the electrons around the atom, one to each of the four sides before pairing. For carbon:

Atomic Lewis structures are an easy way to predict how many bonds an element will often make. The number of single electrons shown on an atomic Lewis structure shows the number of bonds the element is likely to form in neutral molecules. This is a general guideline but many exceptions exist. The bonding patterns in polyatomic ions will often vary from the prediction made based on the neutral atomic structure.

Lewis Structures for Molecules and Polyatomic Ions

To draw a Lewis structure, we will follow a similar process. We will use carbonate (CO₃^2-) as our example.

1. Determine the central atom. This is typically the less electronegative atom or the one listed first in the formula (but not always). Hydrogen is never the central atom! For CO₃^2-, carbon is less electronegative and listed first, so we will assume it is the central atom.
2. Write out the atoms, with the central atom in the middle and the other atoms arranged around it in space. For carbonate, we can show a carbon atom with one oxygen to the right of the carbon, one oxygen to the left of the carbon and one oxygen above the carbon:

3. Determine the total number of electrons in the molecule/ion.
   1. Add all the electrons in the valence shells of the atoms. For carbonate, 4 electrons come from the carbon and 6 electrons come from each oxygen. Thus, the total number of valence electrons in carbonate is 4 electrons + 3 \(*\) 6 electrons = 22 electrons.
   2. Add or subtract electrons from the neutral atoms based on the charge of the ion. For neutral molecules, omit this step. For carbonate, the 2- charge indicates that two more electrons must be added. (Recall that electrons have a negative charge.) The total number of electrons in the carbonate Lewis structure will be 22 electrons + 2 electrons = 24 electrons.
4. Distribute the electrons, ensuring that all outside atoms have 8 electrons either shared or as lone pairs around it. The exceptions to 8 electrons are H, which has only 2 electrons; Be, which usually only has 4 electrons; and B, which often has 6 electrons. For carbonate, each oxygen atom has four pairs of electrons, with each oxygen sharing one pair of electrons with the carbon as shown:

5. See if the central atom has 8 electrons (an octet) around it. The exceptions for Be and B still apply. When we look at carbonate, we see that the C only has 6 electrons around it, due to three oxygens each sharing one pair of electrons.
6. Share more electrons to fill the octet for the central atom. Remember to remove a lone pair of electrons from the atom that is being shown to share more electrons. If more than one atom could share electrons, pick one to share. For carbonate, any of the oxygens could share more electrons (top, left, or right). It does not matter which oxygen is picked to share more electrons, so we will pick the top one.

7. Count the electrons again to make sure that the correct number are shown. In the structure above in step 6, we have shown 24 electrons.
8. To simplify the structure, replace the bonding electrons with one line for each pair. The lone pairs remain as dots. For carbonate, there will four lines between carbon and the oxygens. One oxygen will have 2 lines between it and carbon, the other two will have one line:

9. For ions, draw brackets around the Lewis structure and indicate the charge. For carbonate, we will use the structure shown in step 8, add brackets, and place the 2- charge in the upper righthand corner: