Lewis Structures

[Jessica]

In the Lewis structure of a molecule or polyatomic ion, valence electrons occur in pairs. There are two kinds of pairs.

1. A shared pair, or bonding pair, exists between two atoms of the compound. It can be represented by a straight line between the bonded atoms.
2. An unshared pair, or lone pair, belongs entirely to one atom. It is represented by a pair of dots on that atom.

RULES

1. Draw a skeleton structure for the molecule or ion by joining the atoms with single bonds.
   
   1. Hydrogen is always an end or terminal atom. It is connected to only one other atom.
   2. The least electronegative atom in the molecule or ion is usually the central atom.
   3. Oxygen is NEVER the central atom unless bonded with fluorine or hydrogen.
2. Count the number of valence electrons. It has to be even.
   
   1. For molecules, simply add up the group numbers of the elements.
   2. For ions, add to the sum the charge on a negative ion and subtract the charge on a positive ion.
3. Determine the number of electron pairs that are available to satisfy the octet rule of each element by dividing the total number of valence electrons in half.
4. Determine the number of electron pairs still available for distribution by subtracting the number of single bonds (drawn in step 1) from the total number of pairs available (calculated in step 3).
5. Place lone pairs around each terminal atoms (except H) to satisfy the octet rule. If pairs are left at this point, assign them to the central atom.
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.
   
   1. If the central atom is two electrons short, move one lone pair to convert a single bond to a double bond.
   2. If the central atom is four electrons short, move two lone pairs to convert a single bond to a triple bond (or two single bonds to two double bonds).
   3. Halogens NEVER multiple bond, and if another structure is possible, oxygen will not triple bond.