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Ionic Compounds: Writing Formulas

Ionic Compounds: Writing Formulas. Empirical Formulas. formulas have smallest whole-number ratio of elements in compound Ionic compounds only written as empirical formulas. Identify: ? empirical ? ionic. H 2 O H 2 O 2 CaF 2 NaCl CH 4 C 2 H 6 NiO FeCl 3. Empirical – NOT ionic.

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Ionic Compounds: Writing Formulas

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  1. Ionic Compounds:Writing Formulas

  2. Empirical Formulas formulas have smallest whole-number ratio of elements in compound Ionic compounds onlywritten as empirical formulas

  3. Identify: ? empirical ? ionic • 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

  4. Formula Unit is 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

  5. Binary Ionic Compounds • Composed of twodifferentelements • positive monatomic metal ion • negative monatomic non-metal ion Note: A binary compound may contain more than 2ionsbut only 2kindsof ions like Al2O3

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

  7. Oxidation States Group A 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 (-) Group B elements (transition metals): • can have more than 1 oxidation state • but are always (+)

  8. Writing Formulas for Binary Ionic Compounds first rule in writing formulas POSITIVE ION FIRST How get the subscripts? start by writing formulas from the ions

  9. Compounds are electrically neutral • Sum of all charges 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 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 charges NOT equal but opposite: • Mg+2 and Cl-1, CROSS and DROP! (# only, forget signs!) • Mg1Cl2 (if subscript is 1, forget it!) MgCl2

  13. 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!

  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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