Unit #1Unit #2Unit #3Unit #4Unit #5Unit #6Unit #7Unit #8Unit #9Unit #10
spacerUNIT # 5  

spacer5.1 Ionic

spacerIonic Bonding in Sodium Chloride
spacerIonic Bonding in Magnesium Chloride
spacerProperties of Ionic Compounds
spacer5.2 Covalent
spacer5.3 Polar Covalent

spacer5.4 Bond

spacer5.5 Electroneg-

spacer5.6 Classification
spacerof Bond Type
spacer5.7 Polarity of
spacer5.8 Writing
spacerWriting formulas containing simple ions
spacerWriting formulas containing  polyatomic ions
spacerUsing parentheses in formula writing
spacer5.9 Information in
spacera Chemical
spacerNumber ratio of atoms
spacerNumber ratio of ions
spacer5.10 Oxidation
spacerAssignment of Oxidation Numbers

spacer1 | 2


5.10 -
Oxidation Numbers

5.10.1 - Assignment of Oxidation Numbers

The rules for assigning oxidation numbers are in the Chemist's tools A chemist's tools.
Click on the icon and follow the eleven rules to assign oxidation number to atoms in a chemical formula.

Let's go through the examples below to see how it's done.

Now let's take a look at sodium nitrate, NaNO3. What is the oxidation number of nitrogen?

First of all, let's remember what we've learnt:

  • sodium belongs to group 1A likes to form a +1 charge ion (section 4.6)
  • if we forget the charge of the nitrate ion, we can figure it out using the chemical formula. We know that NaNO3 is a compound and compounds must be neutral. Since sodium ion is +1, the nitrate ion (NO3-) must have a -1 charge because we see a 1:1 ratio of sodium ion to nitrate ion in the chemical formula (see example in section 5.9)

To determine the oxidation number of nitrogen, we apply

Rule #6: Oxygen is usually assigned an oxidation number of -2 for oxides. In this case, we have three oxygen atoms in NO3-.
Rule #9: The sum of oxidation numbers of an ion or complex ion is the same as the charge on that ion.

Let the oxidation number of nitrogen be represented as N. Set up the equation and solve for N.

N + 3 (-2) = -1
N = +5

The apparent charge on nitrogen is +5.

Section 6.3
Oxidation Numbers. Calculating Oxidation Numbers ..p121


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