Forming New Substances Chapter 9 - 2 Part 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.

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. Ca2+ Cl1- 4. Fill in the formula. CaCl2

Continued If they have the opposite charges or can be reduced, then they cancel each other out. For example: Li 1+ F1- = LiF Al 3+ P 3- = Al3P3 or AlP Ca2+ C4+ = Ca4C2 or Ca2C Pb4+ S2- = Pb2S4 or PbS2

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

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

LET’S TRY IT!