1. Forming New Substances
Chapter 9 - 2
Part 2

2. Writing Formulas - Ionic Compounds
The formula must have subscripts that cause the charges of the ions to cancel out making the compound neutral.
An oxidation number is a positive or negative number.

3. Crisscross Method
1. Write the chemical symbols for each element in the compound (positive 1st)
Ex: compound of calcium & chlorine Ca Cl
2. Write the oxidation numbers above the symbols Ca Cl
3. Crisscross the oxidation numbers making them subscripts. Ca Cl1-
4. Fill in the formula. CaCl2

5. Continued
If they have the same charges or can be reduced, then they cancel each other out.
For example:
Li F = LiF
Al 3+ P = Al3P3 or AlP
Ca C = Ca4C2 or Ca2C
Pb S = Pb2S4 or PbS2

6. Let’s Practice 1. Be N 2. Ca F 3. Sr P 4. Ba Pb 5. Al I 6. K O
7. Mg Br
8. Na Cl

7. NAMING THE FORMULAS
Ionic compounds do not use prefixes like covalent compounds do.
The second name ends in “ide” just like covalent do.
Examples: Be3N2 Beryllium Nitride or CaF2 is Calcium Fluoride

8. LET’S TRY IT!