Ionic Compounds: Writing Formulas

Empirical Formulas = smallest whole-number ratio of elements Ionic compounds: empirical formulas only

Identify the empirical formulas H2O H2O2 CaF2 NaCl CH4 C2H6 FeCl3 Empirical Molecular Empirical (Ionic) Empirical (Ionic) Empirical Molecular Empirical (Ionic)

Binary Ionic Compounds Composed of two different elements Positive monatomic metal ion Negative monatomic nonmetal ion Note: binary compound may contain more than 2 ions but only 2 kinds of ions like Al2O3

Oxidation Number O-2 Ca+2 Na+1 Fe+2 and Fe+3 Charge of a monatomic ion Some elements form only 1 ion Some elements can form more than 1 ion O-2 Ca+2 Na+1 Fe+2 and Fe+3

Oxidation State of Metal Elements Group 1 ions always +1 Group 2 ions always +2 MOST group 13 ions are +3 Groups 14, 15, 16, and 17: multiple oxidation states Transition metals (Group B) can have more than 1 oxidation state

Writing Formulas: Binary Ionic Compounds first rule is: POSITIVE ION FIRST (metal) How do you get subscripts? start by writing the ions

Compounds are neutral: Sum of all charges must = 0 (+) and (-) charges must = each other

Charges: Equal but Opposite Na+1 and Cl-1: NaCl +1 + (-1) = 0 Mg+2 and O-2: MgO +2 + (-2) = 0 Al+3 and P-3: AlP +3 + (-3) = 0 Rule: write the symbols, (+) FIRST!

Try a few formulas: Li+1 and I-1 Ca+2 and O-2 Al+3 and N-3 K+1 and F-1 Ba+2 and S-2 LiI CaO AlN KF BaS

Charges: Opposite but NOT equal Criss-Cross Method: Mg+2 and Cl-1, CROSS and DROP! (# only, forget signs!) Mg1Cl2 (if subscript is 1, forget it) MgCl2 (1 Mg+2 and 2 Cl-1) [check math: +2 + -1 + -1 = 0]

Try a few formulas: Ca+2 + Cl-1 Na+1 + O-2 Cs+1 + S-2 Al+3 + Cl-1 Al+3 + Se-2 Mg+2 + F-1 K+1 + N-3 CaCl2 Na2O Cs2S AlCl3 Al2Se3 MgF2 K3N