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Oxidation # & Writing Formulas

Learn how oxidation numbers reflect electron behavior and how to correctly write chemical formulas for ionic compounds using the criss-cross method. Understand the principles of gaining and losing electrons.

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Oxidation # & Writing Formulas

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  1. Oxidation # & Writing Formulas

  2. Oxidation Numbers • A positive (+) or negative (-) number • Shows the number of electrons gained, lost, or shared when forming a compound • Sometimes called the “CHARGE” of the atom • An atom’s oxidation number tells us how many electrons it needs to fill an octet

  3. I. Oxidation Numbers d. GAINING electrons results in a NEGATIVE Oxidation Number. Non-metals gain electrons i. Sulfur (Group 16) 1. Gains 2 electrons to reach an octet 2. Becomes S-2 e. LOSING electrons results in a POSITIVE Oxidation Number. Metals lose electrons. i. Calcium (Group 2) 1. Loses 2 electrons to reach an octet 2. Becomes Ca+2

  4. g. Oxidation Numbers +4or -4 +1 +2 +3 -3 -2 -1

  5. II. Exchanging Electrons: LITHIUM a. In Group 1 (the Alkali Metals) b. Has 1 valence electron c. To become stable: i. Gives 1 electron to another atom d. Now has i. 3 Protons ii. 2 Electrons (one more protons (+) than electrons) iii. Charge of +1

  6. III. Chemical Formulas a. Molecular Formulas i. Shows the elements in a compound and the numbers of each element b. Subscript i. Shows the ratio of how many atoms of an element are present in a compound ii. Small number at the bottom right of the element

  7. c. Chemical Formula Example: Water i. H2O 2 atoms of Hydrogen 1 atom of Oxygen subscript

  8. d. Chemical Formula Example: Glucose i. Molecular Formula: C6H12O6 ii. 6 atoms of Carbon iii. 12 atoms of Hydrogen iv. 6 atoms of Oxygen

  9. IV. Chemical Bonds a. ION i. An atom that has a positive or negative charge resulting from the GAIN or LOSS of electrons b. IONIZATION i. The process of becoming an ion

  10. IV. Chemical Bonds c. CATION i. A Positive Ion ii. Metals iii. CATion = PAWSitive d. ANION i. A Negative Ion ii. Nonmetals Ca +2 O -2

  11. V. Ionic Bond a. The FORCE of ATTRACTION between the OPPOSITE CHARGES of the ions in an ionic compound (OPPOSITES ATTRACT) b. ALWAYS form between METALS and NONMETALS i. Metals LOSE electrons ii. Nonmetals GAIN electrons

  12. Ionic Bond & Lewis Dot i. Can show the transfer of electrons Na1+ + Cl 1-Na Cl 1+ 1-

  13. VI. Ionic Compounds • Have both CATIONS and ANIONS so NET CHARGE = Zero • Example Na+1 + Cl-1 NaCl +1 + -1 = 0 c. Properties i. Dissolve in Water iii. Strong Bonds (Electrostatic)

  14. d. Which Groups are Most Likely to Bond Easily i. Alkali Metals & Halogens (SALT FORMERS) • Na & Cl • K & Br ii. Alkaline Earth Metals & the Oxygen Group • Mg & O • Ca & S Example: HALITE

  15. e. SIDE NOTE: Ionic Compounds & Conducting Electricity i.Ionic bonds break down in water, allowing the cations (+) and anions (-) to move about in the water ii. The cations and anions carry the charge

  16. F. Writing Ionic Formulas • Write the symbol of the element that has the positive oxidation number (Metals & H) • Write the symbol of the element that has the negative oxidation number (Nonmetals) • Use the CRISS-CROSS Method to make the sum of the oxidation numbers = zero • Simplify if there is a common factor

  17. Ionic Compound Example POTASSIUM SULFUR K +1 S-2 K +1 +2 -2 You needed 2 potassiums and 1 sulfur to equal zero so K2S **NOTE: Do not write the Subscript “1”**

  18. Ionic Compound Example POTASSIUM SULFUR K +1 S-2 K2 S1 K2S **NOTE: Do not write the Subscript “1”**

  19. Ionic Compound Example MAGNESIUM CHLORINE Mg +2 Cl-1 Mg1 Cl2 MgCl2 **NOTE: Do not write the Subscript “1”**

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