1. 22.3 – Writing formulas and Naming Compounds

2. Objectives Explain how to determine oxidation numbers
Write formulas and names for ionic compounds

3. Binary Ionic Compounds
A compound composed of 2 elements
To write a chemical formula for a binary compound, you need to know:
which elements are involved
how many electrons they lose or gain to become stable

4. Oxidation Numbers - Transition Elements
Because they can have multiple oxidation numbers, we have to express that number in the name with a Roman Numeral
For example, the oxidation number of iron in
iron (III) oxide is 3+.

5. Write Formulas for these
Potassium ion
Magnesium ion
Copper (II) ion
Chromium (VI) ion
Barium ion
Mercury (II) ion

6. Ionic Compounds are NEUTRAL
Although the individual ions in a compound carry charges, the compound itself is neutral
A formula must have the right number of positive ions and the right number of negative ions so the charges balance

7. Ca+2 Cl-1 would have a +1 charge. Need another Cl-1 Ca+2 Cl 1- Cl 1-
Writing Formulas
Write the formula for calcium chloride.
Calcium is Ca+2
Chloride is Cl-1
Ca+2 Cl-1 would have a +1 charge.
Need another Cl-1
Ca+2 Cl 1- Cl 1-
CaCl2

8. Step By Step – Writing Formulas
1. Write the symbol of the element (or polyatomic ion) with the positive oxidation number or charge
2. Write the symbol of the element (or polyatomic ion) with the negative oxidation number or charge
3. The charge (without the sign) of one ion becomes the subscript of the other ion.
Reduce the subscripts to the smallest whole numbers that retain the ratio of ions.

9. Criss-Cross Method

10. Write the formulas for these
Lithium sulfide
Tin (II) oxide
Tin (IV) oxide
Magnesium fluoride
Copper (II) sulfate
Iron (III) phosphide
Gallium nitrate
Iron (III) sulfide
Barium nitrate

11. In-Class Assignment/Homework
22.3a Criss Cross WS
22.3a Criss-Cross practice