The Lewis structure for a molecule or polyatomic ion shows the distribution of the valence electrons available on that species as bonding and nonbonding electron pairs. Many common species have a central atom bonded to one or more other atoms (usually halogen or hydrogen or oxygen).
To draw a Lewis structure:
Note that while both halogen and oxygen outer atoms are surrounded by a maximum of eight electrons, the central atom may have more than eight electrons for elements in the third row and beyond in the periodic table.
To draw a Lewis structure:
- Count available electrons.
The number contributed by each atom is the last digit of its Group number in the periodic table except that H contributes 1.
- Connect bonded atoms by single bonds (involving one electron pair).
Subtract two electrons per bond from the total available electrons.
- Complete octets (an octet is eight electrons) at outer atoms other than hydrogen.
The singly-bonded atoms each already have two electrons. This is the maximum number for hydrogen.
Other outer atoms in Lewis structures have eight electrons (an octet).
Place three nonbonding electron pairs at each outer atom other than H.
Subtract the number of electrons used from the total.
- Place any remaining electron pairs at the central atom.
EXAMPLE:
| Group 14 C (4 × 1) | 4 |
| H (1 × 1) | 1 |
| Group 17 Cl (7 × 3) | 21 |
| Step 1: Electrons available | 26 |
| Step 2: Electrons used in bonds (4 × 2) | –8 |
| Step 3: Electrons used to complete octets at Cl (6 × 3) | -18 |
| Step 4: Electrons to be placed at central atom | 0 |
Note that while both halogen and oxygen outer atoms are surrounded by a maximum of eight electrons, the central atom may have more than eight electrons for elements in the third row and beyond in the periodic table.