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 1 st ) 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 2+ Cl 1- 4. Fill in the formula. CaCl 2

5. Continued If they have the same charges or can be reduced, then they cancel each other out. For example: Li 1+ F 1- = LiF Al 3+ P 3- = Al 3 P 3 or AlP Ca 2+ C 4+ = Ca 4 C 2 or Ca 2 C Pb 4+ S 2- = Pb 2 S 4 or PbS 2

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: Be 3 N 2 Beryllium Nitride or CaF 2 is Calcium Fluoride

8. LET’S TRY IT!