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Understanding Ionic Compounds: Writing Empirical Formulas and Formula Units

This guide provides a comprehensive overview of ionic compounds, focusing on how to write empirical formulas and identify formula units. Ionic compounds consist of positively charged metal ions and negatively charged nonmetal ions, always adhering to the principle of electrical neutrality, where the sum of charges must equal zero. We will explore the rules for writing formulas, including the criss-cross method, and understand the significance of oxidation states. Examples of common ionic compounds will be examined to solidify understanding.

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Understanding Ionic Compounds: Writing Empirical Formulas and Formula Units

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  1. Ionic Compounds:Writing Formulas

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

  3. 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

  4. 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

  5. Binary Ionic Compounds • Composed of two different elements • Positive monatomic metal ion • Negative monatomic nonmetal 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 Al+3 and Ag+1 Oxidation Number or State • charge of monatomic ion • has right superscript • Some elements form only 1 ion • Some elements can form more than 1 ion

  7. 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 (+)

  8. 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

  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 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

  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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