AP Chemistry
Lewis Structures Help Sheet
In the Lewis Structure of a molecule or polyatomic ion, valence electrons occur in pairs. There are two kinds
of pairs.
1. A bonding pair (BP) exists between two atoms of the compound. It can be represented by a straight
line between the bonded atoms.
2. A lone pair (LP) belongs entirely to one atom. It is represented by a pair of dots on that atom.
Steps in Drawing Lewis Structures
1. Draw a skeleton structure for the molecule or ion by placing an atom in the center and the terminal
atoms around the center atom
a. Hydrogen is always an end or terminal atom. It can only have one single bond.
b. The least electronegative atom in the molecule or ion is usually the central atom (it is usually the
atom that only appears once).
c. Oxygen is rarely the central atom (unless it is bonded with fluorine or hydrogen).
2. Count the number of valence electrons available for your structure.
a. For molecules, simply add up the group numbers of the elements.
b. For ions, add to the sum of the group numbers the charge on a negative ion and subtract the
charge on a positive ion.
3. Connect the terminal atoms to the central atom by placing a pair of electrons (a single bond) between the
outer atoms and the central one.
4. Determine the number of electrons still available for distribution. To do this, subtract the number of
electrons used in step 3 from the total number of electrons available (calculated in step 2).
5. Place lone pairs around each terminal atom (except H) to satisfy the octet rule. If pairs are left at this
point, assign them to the central atom. (If the central atom is from the third or higher period, it can have
more than 8 electrons surrounding it. This is called an expanded octet and will be discussed in greater
detail later.)
6. If the central atom is not yet surrounded by 8 electrons, multiple bonds must be formed by moving one
or more terminal lone pairs to a bonding location.
a. If the central atom is two electrons short, move one lone pair to convert a single bond to a
double bond.
b. If the central atom is four electrons short, move two lone pairs to convert a single bond to a
triple bond.