The first step in drawing a Lewis structure for a molecule or polyatomic ion is to count the number of valence electrons available in that collection of atoms.
The number of valence electrons available for a particular atom depends on the Group in which the atom appears in the periodic table and is equal to the last digit of the Group Number.
The number of electrons available for a molecule is the sum of the valence electrons for all of its atoms.
Consider H2O
H—O—H
Use the guidelines in the following examples to draw a Lewis structure for any collection of atoms.
The number of valence electrons available for a particular atom depends on the Group in which the atom appears in the periodic table and is equal to the last digit of the Group Number.
The number of electrons available for a molecule is the sum of the valence electrons for all of its atoms.
Consider H2O
H—O—H
| 2 H | 2 electrons |
| 1 O (Group 16) | 6 electrons |
| total electrons available | 8 electrons |
The Lewis structure shows that four of the eight electrons are in single bonds (each line between atoms represents two bonding electrons) and four are nonbonding electron pairs.
Use the guidelines in the following examples to draw a Lewis structure for any collection of atoms.