Chapter 7: Chemical Formulas and Chemical Compounds

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Chapter 7 Chemical Formulas and Chemical
Compounds

• Section 2 Oxidation Numbers

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Overview

• We will list the rules for assigning oxidation
  numbers.
• We will be able to give the oxidation number for
  all the elements in a compound.
• We will name binary molecular compounds using
  oxidation numbers and the stock system.

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Oxidation Numbers

• A way to indicate the distribution of electrons
  among bonded atoms in a molecular compound or a
  polyatomic ion.
• Also known as an atoms oxidation state.
• This is not the same as an atoms charge!
• Oxidation numbers are helpful in some cases in
  naming a compound.
• Later we will use them in balancing equations.

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How to Assign Oxidation Numbers

• Atoms by themselves or bonded to themselves have
  an oxidation state of 0.
• Hydrogen has an oxidation number of 1 and when
  bonded to a metal -1.
• Fluorine is always a -1.
• Oxygen is -2 unless in H2O2 (-1) or OF2 (2).
• The most electronegative atom has an oxidation
  number equal to its charge as an ion. (Closest to
  F)
• The charge of an individual ion is its oxidation
  number.
• The sum of the oxidation numbers must equal the
  charge of the compound.

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Binary Molecule Examples

• HF
• Both have fixed values H 1 F -1
• Cl2
• Bonded to Self Cl 0
• CS2
• Sulfur is more electronegative S -2
• There are two sulfur atoms so their oxidation
  totals to -4. The carbon atom must equal this
  charge to make the molecules total oxidation
  0. C 4

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Polyatomic Ion Examples

• SO4-2
• Oxygen is more electronegative fixed value. O
  -2
• There are four oxygen atoms, so the total
  negative charge due to oxygen is -8.
• The total charge needs to equal -2 so sulfur must
  account for that. S 6
• SO3-2
• Oxygen is more electronegative fixed value. O
  -2
• There are three oxygen atoms, so the total
  negative charge due to oxygen is -6.
• The total charge needs to equal -2 so sulfur must
  account for that. S 4

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Molecular Compounds w/ Polyatomic Ions

• H2CO3
• CO3 has a -2 charge so that is its oxidation
  number.
• Hydrogen is always 1. H 1
• Oxygen is the most electronegative atom fixed
  value. O -2
• Total oxidation number of the three oxygen atoms
  is -6
• So the oxidation number of carbon and -6 need to
  equal -2. C 4

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Using Oxidation States to Name

• We already do this in the stock system for ionic
  compounds.
• The prefix system for molecular compounds can be
  modified using the oxidation numbers to the stock
  system.
• NCl3
• Nitrogen trichloride
• Nitrogen (III) chloride