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76 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 8
predicted by means of a Lewis structure. The following steps can be
used in drawing Lewis structures.
1. Predict the location of certain atoms. Hydrogen is
always a terminal atom. The atom with the least
attraction for electrons is the central atom.
2. Find the total number of electrons available for
bonding (valence electrons). If the structure is to represent a positive or negative polyatomic ion, the ion
charge must be subtracted or added, respectively.
3. Divide the total number of available electrons by 2 to
obtain the number of bonding pairs.
4. Place one bonding pair between the central atom and
each terminal atom.
5. To find the total number of lone pairs and pairs available for multiple bonding, subtract the number of
bonding pairs used in step 4 from the number of
bonding pairs determined in step 3. Place lone pairs
around the terminal atoms to satisfy the octet rule.
Assign remaining pairs to the central atom.
6. If the central atom is not surrounded by four electron
pairs, convert one or two lone pairs on the terminal
atoms to a double or triple bond to the central atom.
Example Problem 8-5
Drawing Lewis Structures
Draw Lewis structures for each of the following.
a. phosphorus trichloride (PCl 3 )
b. sulfate ion (SO 4
2Ϫ )
Apply the six steps in each case.
1. The central atoms, which have the least attraction for
shared electrons, must be:
a. P (P is in group 15, whereas S is in group 17)
b. S (S is below O in group 16)
2. The total available electrons for bonding are:
a. (5 from P) ϩ (3 ϫ 7 from Cl) ϭ 26
b. (6 from S) ϩ (4 ϫ 6 from O) ϩ 2 (from charge) ϭ 32