Ionic Compounds: Writing Formulas

Empirical Formulas = smallest whole-number ratio of elements in compound Ionic compounds only have empirical formulas

Identify the empirical formulas H2O H2O2 CaF2 NaCl CH4 C2H6 NiO FeCl3 Empirical – NOT ionic Molecular – NOT ionic Empirical, Ionic Empirical, Ionic Empirical – NOT ionic Molecular – NOT ionic Empirical, Ionic Empirical, Ionic

Formula Unit = simplest ratio of ions in ionic compound Examples: KBr is formula unit potassium ions & bromide ions are in a 1-to-1 ratio AlCl3 is formula unit Aluminum & chloride ions in 1-to-3 ration

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

Oxidation Number or State Na+1 charge of monatomic ion has right superscript Some elements form only 1 ion Some elements can form more than 1 ion Ca+2 O-2 Al+3 and Ag+1 Fe+2 and Fe+3

Oxidation States Group 1 ions always +1 Group 2 ions always +2 MOST group 13 ions are +3 Groups 14, 15, 16, and 17 have multiple oxidation states both (+) and (-) Transition metals (Group B) can have more than 1 oxidation state but are always (+)

Writing Formulas for Binary Ionic Compounds first rule in writing formulas for ionic compounds is POSITIVE ION FIRST But how do you get the subscripts? start by writing formulas from the ions

Compounds are electrically neutral Sum of all charges in compound should be 0 (+) and (-) charges must = each other

Equal but Opposite Charges 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 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

Criss-Cross Method If charges are not equal but opposite: Mg+2 and Cl-1, CROSS and DROP! (# only, forget signs!) Mg1Cl2 but if subscript is 1, forget it! MgCl2 means 1 Mg+2 and 2 Cl-1

Check the Math MgCl2 means 1 Mg+2 and 2 Cl-1 (1)(+2) + (2)(-1) = +2 + -2 = 0 charges MUST add up to zero!

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

Of course, it gets more difficult Potassium (K) and Fluorine (F) Zinc (Zn) and Iodine (I) Sodium (Na) and Oxygen (O) Magnesium (Mg) and Oxygen (O) Aluminum (Al) and Oxygen (O) Calcium (Ca) and Bromine (Br) Cesium (Cs) and Iodine (I) Silver (Ag) and Sulfur (S) KF ZnI2 Na2O MgO Al2O3 CaBr2 CsI Ag2S