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

In section 4.6, we learned the specific charge of ions formed by representative elements. In section 5.1.1 we learned that when an atom loses an electron to form a cation, the atom is said to be oxidized. When an atom gains an electron to form an anion, the atom is said to be reduced.

We can assign oxidation number to each element in a compound to keep track of electrons in oxidation and reduction processes.
Oxidation numbers can be zero, positive or negative numbers. For this course, you'll only encounter oxidation numbers as integers. Aside from the simple ions in the compound, oxidation numbers bear no real physical meaning.

In polyatomic compounds which contain polyatomic ions*, elements in polyatomic ions do not have measurable ionic charge. For example, in sodium nitrate, NaNO3,

  • the sodium ion carries a +1 charge (section 4.6)
  • the nitrate carries a -1 charge (nitrate is a polyatomic ion*)
  • the nitrogen in the nitrate ion does not have any ionic charge since it is covalently bonded to oxygen atoms. However, we can determine an "apparent charge" that is associated with nitrogen.

Since we know that a compound must be neutral in charge, we follow a set of rules to assign the oxidation number to nitrogen. We would determine that the apparent charge on nitrogen is +5.

Let's take a look at how I arrived at an oxidation number of +5 for nitrogen on the next page.


* Click on Chemist's Tool and find the icon to learn the polyatomic ions.

Section 6.3
Oxidation Numbers. Calculating Oxidation Numbers ..p121


All contents copyrighted © 1996-2006
British Columbia Institute of Technology
Chemistry Department - 3700 Willingdon Avenue
Burnaby, B.C. Canada V5G 3H2