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Ionic Compounds: Writing Formulas

Ionic Compounds: Writing Formulas. Empirical Formulas. smallest whole-number ratio of elements in the compound Ionic compounds have empirical formulas only. Identify the empirical formulas. H 2 O H 2 O 2 CaF 2 NaCl CH 4 C 2 H 6 NiO FeCl 3. Empirical. Molecular. Empirical and Ionic.

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Ionic Compounds: Writing Formulas

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  1. Ionic Compounds:Writing Formulas

  2. Empirical Formulas • smallest whole-number ratio of elements in the compound • Ionic compounds have empirical formulas only

  3. Identify the empirical formulas • H2O • H2O2 • CaF2 • NaCl • CH4 • C2H6 • NiO • FeCl3 Empirical Molecular Empirical and Ionic Empirical and Ionic Empirical Molecular Empirical and Ionic Empirical and Ionic

  4. Formula Unit • The simplest ratio of the ions in an ionic compound • KBr • potassium ions & bromide ions are in a 1-to-1 ratio • is the formula unit • For ionic compounds, the formula mass is based on the formula unit

  5. Binary Compounds • Composed of two elements • Positive monatomic metal ion • Negative monatomic nonmetal ion (Monoatomic ion is a one-atom ion) Note: A binary compound may contain more than 2ionsbut only 2kindsof ions like Al2O3

  6. Na+1 Ca+2 O-2 Fe+2 and Fe+3 Oxidation Number or State • Charge of a monatomic ion • Given by right superscript • Some elements form only 1 ion • Some elements can form more than 1 ion • Try to find patterns

  7. Oxidation State of Selected Elements • 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 +

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

  9. Writing Formulas for Binary Compounds • Compounds are electrically neutral • Sum of all in compound should be 0 • (+) and (-) charges must = each other

  10. 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 the symbols, (+) FIRST!

  11. 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

  12. Criss-Cross Method • When the charges don’t cancel out immediately: • Mg+2 and Cl-1, CROSS and DROP! (# only, forget signs!) • Mg1Cl2 but if the subscript is 1, forget it! • MgCl2 means 1 Mg+2 and 2 Cl-1

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

  14. Try a few formulas: CaCl2 • 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 Na2O Cs2S AlCl3 Al2Se3 MgF2 K3N

  15. Of course, it gets more difficult KF • 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) ZnI2 Na2O MgO Al2O3 CaBr2 CsI Ag2S

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