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Ionic Compounds: Writing Formulas
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Empirical Formulas = smallest whole-number ratio of elements
Ionic compounds: empirical formulas only
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Identify the empirical formulas
H2O H2O2 CaF2 NaCl CH4 C2H6 FeCl3 Empirical Molecular Empirical (Ionic) Empirical (Ionic) Empirical Molecular Empirical (Ionic)
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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
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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
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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
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Writing Formulas: Binary Ionic Compounds
first rule is: POSITIVE ION FIRST (metal) How do you get subscripts? start by writing the ions
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Compounds are neutral:
Sum of all charges must = 0 (+) and (-) charges must = each other
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Charges: Equal but Opposite
Na+1 and Cl-1: NaCl (-1) = 0 Mg+2 and O-2: MgO (-2) = 0 Al+3 and P-3: AlP (-3) = 0 Rule: write the symbols, (+) FIRST!
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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
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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: = 0]
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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
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