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23 - Hoffman - Odd electron problems
Added by Mary Blackwell, last edited by Emily Hoffman on Nov 05, 2007

Molecules or ions that contain an odd number of total valence electrons are called radicals. Because of the odd number of electrons, it is not possible for all of the electrons to be in pairs. Therefore, they must disobey the octet rule.

Radicals are relatively rare and the unpaired electrons usually cause them to be very reactive and unstable.

For a more in depth look at the properties of radicals, please see http://en.wikipedia.org/wiki/Radical_%28chemistry%29

In the case of a radical, you must consider electronegativity and formal charge in order to place a charge on an individual atom.

The following are examples of odd-electron molecules:

Example 1

The nitrogen dioxide, NO2, molecule has 17 valence electrons.

The lewis structure of the molecule reveals that it is polar, with a bent shape, and also has resonance forms.

The lewis structure of NO2 can also be drawn as the following:

Images of resonance forms of NO2 (from http://www.madsci.org/posts/archives/2000-02/951452623.Ch.r.html)

Even though the formal charges show that the atoms are "happier" in the second lewis structure, the single odd electron is placed on the Nitrogen atom in the first lewis structure because oxygen has a greater electronegativity than nitrogen. Oxygen has more power to fulfill it's octet. Therefore the first image is correct.

NO2 is very reactive because of the unpaired electron on the nitrogen atom. It readily forms dinitrogen tetraoxide, N2O4, when the odd electrons on the nitrogen atom of two NO2 molecules pair up in a process called dimirization.

Example 2

Another example of an odd-electron molecule is nitrogen monoxide, or nitric oxide.

NO has 11 valence electrons.

Image of lewis structure of NO (from http://userpages.umbc.edu/~budzicho/Chp11note2.html)

The nitrogen is stuck with the odd electron (and an incomplete octet) because oxygen is once again more electronegative than nitrogen and has more power to complete its own octet.

Example 3

Chlorine dioxide, ClO2, is another molecule with an odd number of valence electrons.

ClO2 has 19 valence electrons and the lewis structure is as follows:

(Image of Lewis Structure created by Emily Hoffman)

Oxygen is more electronegative than Chlorine, so it is more able to have a complete octet. Chlorine does not fulfill the octet rule, but goes beyond having 8 valence electrons. This is ok because Chlorine can do this. The formal charge on each atom in this molecule is 0.


To understand Electron Impact Ionization and how it leads to odd-electron ions, please refer to http://www-chem.harvard.edu/mass/tutorials/ei.html. The site also includes a nice applet.

Other Sources of Information:


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