1. Ion Charge and the Formulas of Ionic Compounds

2. Recall Compounds are formed when
each atom in the compound attempts to have a ‘full’ outer shell of valence electrons
E.g. NaCl

3. Ionic Compounds
Form between a
metal and a nonmetal

4. Combining capacity (Ion Charge)
Located on the top right hand corner of an elements box on the periodic table.
Definition –
The ability of an element to combine with another element.
Related to its ability to donate and accept electrons

5. For metals the combining capacity is usually
positive and for non-metals it is negative.
When ionic compounds form, every electron that is given up by a metal atom must be accepted by a non-metal atom.
If the elements have equal but opposite ion charges, then they will combine in the ratio 1:1.
Ex. NaCl

6. What about Al3+ S2-
If the two elements have unequal and opposite ion charges, then the elements will combine in a ratio so that the total number of electrons transferred equals the total number of electrons accepted.

7. Example – Aluminum sulfide
Aluminum – Al3+
Combining Capacity = +3
Sulfur– S2- -2
Ratio of combining capacities =
3 sulfurs for every 2 aluminums.

8. Short Cut - Criss –Cross (Drop and Swap) Method
Example: Putting aluminum and sulfur together (aluminum sulfide)
1. Identify each element and its combining capacity. Ex. Aluminum sulfide= Al3+ S2-
2. Drop and swap the numbers from the combining capacity
(Criss-cross the combining capacity)
. Ex. Al3+ S2-
3. Put the symbols together and get rid of the charge values.
Al2S3

9. Al2S3
(Notice that 2Al3+ = 6+ and 3S2- = 6-; thus, compound charge is neutral (0))

10. Putting calcium and oxygen together (calcium oxide).
Identify each element and its combining capacity. Ex. Ca2+ and O2-.
Criss cross charges. (Drop and Swap)
Ex.
Put the symbols together, get rid of the charge symbols and values.
Ca2O2
Reduce to the lowest common multiple (note-you do not write the one)
CaO

11. What about Polyatomic Ions?
Do the same thing!
Remember to make sure that the number of polyatomic ions when swapped is outside the brackets
Ex. Aluminum and sulfate

12. Practice Write the formulas for the compounds formed
in each of the following:
(a) silver and sulfur
(b) magnesium and chlorine
(c) zinc and bromine
(d) calcium and nitrogen
(e) calcium and nitrate

13. Multivalent
Many elements are multivalent (they have more than one charge)
For these, you will see a Roman numeral to tell you which to use.
Ex. Iron (II) means to use 2+
Manganese (III) means to use 3+
See page 173
(c) McGraw Hill Ryerson 2007

14. Try these:
(f) cobalt(III) and oxygen (g) copper(I) and nitrate