15.4: Lewis Structures: Counting Valence Electrons

How-to: Constructing Lewis electron structures

1. Determine the total number of valence electrons in the molecule or ion.

• Add together the valence electrons from each atom. (Recall that the number of valence electrons is indicated by the position of the element in the periodic table.)
• If the species is a polyatomic ion, remember to add or subtract the number of electrons necessary to give the total charge on the ion.

For CO32−, for example, we add two electrons to the total because of the −2 charge.

2. Arrange the atoms to show specific connections.

• When there is a central atom, it is usually the least electronegative element in the compound. Chemists usually list this central atom first in the chemical formula (as in CCl4 and CO32−, which both have C as the central atom), which is another clue to the compound’s structure.
• Hydrogen and the halogens are almost always connected to only one other atom, so they are usually terminal rather than central.

3. Place a bonding pair of electrons between each pair of adjacent atoms to give a single bond.

• In H2O, for example, there is a bonding pair of electrons between oxygen and each hydrogen.

4. Beginning with the terminal atoms, add enough electrons to each atom to give each atom an octet (two for hydrogen).

• These electrons will usually be lone pairs.

5. If any electrons are left over, place them on the central atom.

• We will explain later that some atoms are able to accommodate more than eight electrons.

6. If the central atom has fewer electrons than an octet, use lone pairs from terminal atoms to form multiple (double or triple) bonds to the central atom to achieve an octet.

• This will not change the number of electrons on the terminal atoms.

7. Final check

• Always make sure all valence electrons are accounted for and each atom has an octet of electrons except for hydrogen (with two electrons).