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What Are Free Radicals? - Definition & Examples

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Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

A free radical refers to a particle that has an unpaired electron. Learn about the definition of free radicals, explore Lewis dot structures, and understand the example of a free radical reaction. Updated: 01/04/2022

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.' 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
LT


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.

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  • 0:04 Definition of a Free Radical
  • 1:05 Lewis Dot Structures
  • 1:33 Free Radical Reaction Example
  • 3:22 Lesson Summary
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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
LD


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
CL


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
FR


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