Free An electron is the negative portion

Free radicals are an important part of chemical reactions. This lesson will go through a free radical reaction, as well as explain the chemistry terms associated with the reaction.

Definition of a Free Radical

Dude, that’s so radical! Okay, so people don’t use ‘radical’ that way these days, but it’s still used quite a bit in the chemistry world, and it doesn’t mean ‘cool’ or ‘awesome.

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‘ Instead, a radical, specifically a free radical, is the term used to describe a particle that has an unpaired electron. An electron is the negative portion of an atom and is found outside the nucleus, or the center of the atom. As you can see from the image on screen of the Lithium atom, electrons can be found close to the nucleus or far away from the nucleus. For this lesson, we’re going to focus on the valence electrons, which are the electrons furthest away from the nucleus, and the electrons most likely to be involved in reactions.

Lithium. Note the center of the atom contains protons and neutrons and is called the nucleus. The electrons are outside of the nucleus. Note the valence electron

Particles, like molecules, often have pairs of electrons. When one isn’t paired up it makes the particle reactive, meaning it’s more likely to be involved in a chemical reaction.

So free radicals are usually reactive. That’s so radical! No? Okay.

Lewis Dot Structures

Before we go back to free radicals, let’s familiarize ourselves with Lewis dot structures, which are dots that represent the valence electrons in a particle. When the dots are paired, the particle is less likely to react. When the dots are unpaired, or alone, the particle is reactive.

You can see that lithium, which has the chemical symbol Li, has one valence electron that is alone, or unpaired. This makes lithium a free radical since its electron doesn’t have a partner.

Lewis dot structure of lithium. Note there is one dot because lithium has one valence electron

Example of a Free Radical Reaction

Now that you have some terminology under your belt, let’s look at some actual free radicals. Let’s say you have two chlorine atoms that are bonded, or stuck, together making a molecule.

Lewis dot structure for two chlorine atoms that are bonded together

Given enough heat, these two chlorines will break apart and each will end up taking their valence electrons with them. You can see from the image that each has seven valence electrons. Now each one has an unpaired valence electron. Uh oh! You know what that means. Yep, they’re free radicals. And they are very reactive and, since electrons don’t like being unpaired, they will try to find another particle to react with.

Each chlorine is a free radical now

Now let’s pretend that there’s some methane hanging around near the chlorine free radicals. Methane is just a carbon atom bonded to four hydrogen atoms. You’ll see in the image that the bonds are represented by a line. Sometimes chemists use a line instead of Lewis dots.

Just realize that each line represents a pair of electrons, or two dots.


Desperate to find a partner for its lone electron, one of the chlorine free radicals ends up taking one of the hydrogen atoms from methane.

The free radical chlorine found an electron

This solves the problem for the free radical chlorine (in fact it’s no longer a free radical because it does not have an unpaired electron), but it turns the former methane molecule into a free radical.

Now the former methane molecule is a free radical

You might have noticed that a free radical created another free radical. So now what? Well the former methane, who is now a free radical, will react with another particle. But, in doing so, it will create another free radical, just like we saw when chlorine reacted with the methane. The moral of the story: one free radical will create another free radical in its attempt to find an electron to make a pair for itself. Eventually two free radicals may find one another, thus ending the cycle, but not necessarily.

Lesson Summary

You were just inundated with a lot of chemistry terms, so let’s take a moment to go over everything so you can have a radical day! Remember, a free radical is a particle that has an unpaired valence electron.

Valence electrons are the electrons that are furthest away from the center of the atom. Remember that the center of the atom is the nucleus. These valence electrons are represented by dots in what is referred to as Lewis dot structures. If there is an unpaired electron, one can likely assume the particle is a free radical.

Those free radicals are very reactive, so they’ll search to find themselves an electron to pair with their lone electron. When the free radical finds and takes an electron, it leaves the electron’s former owner with an unpaired electron, thus creating another free radical.


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