The Electron Shell

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  • 0:06 Electron Shell
  • 1:39 Energy Levels
  • 6:10 Valence Electrons
  • 8:36 Summary
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Lesson Transcript
Instructor: Greg Chin
You may be familiar with the role of electrons in electrical devices, but did you know that electrons also determine the chemical reactivity of everything around you?

We learned already that electrons are negatively charged subatomic particles. We also talked about how the electrons are found in clouds around the nucleus. You've probably seen a lot of symbols for atoms, probably something like this where the electrons are represented as having specific orbits. What actually is the case is that the electron cloud is a prediction of the area in which the electrons can be found.

Illustration of an electron cloud with two electrons
Electron Cloud Illustration

So, if we have our nucleus in the middle and we think about this circle as being a 3-dimensional sphere around the nucleus, we can say that any given time, if I was to take a snapshot of this atom, that the electron would be found somewhere in this sphere or this cloud around the nucleus. But I couldn't be sure exactly where it is. If I were to take different photos at different times, then the electron would be in an unpredictable, nonrandom place somewhere in that cloud.

The electron cloud is just giving us a prediction of the area in which the electrons can be found. There's also energy associated with the electron clouds. The electron clouds are also known as electron shells, and they're also known as energy levels.

Let's just make a representation in 2-D of an atom. If we have our nucleus here, I can draw one energy level like this, and I can put an electron here. Now, think about subatomic particles in terms of their charge. The electron has a negative charge, and the protons in the nucleus have a positive charge. Negative and positive things attract each other.

If I had two powerful magnets here, and one end has a positive charge, and one end has a negative charge, they're going to want to move together. Now, the fact that the electron is orbiting out here outside of the nucleus, that it's found out here away from the positive charge, means that it's moved away, farther away than that charge would probably prefer to be.

If I was to take this negative magnet and move it across the table and if these are powerful magnets, then I'm going to have to exert some force to keep this negative magnet away from the positive one. Otherwise, it's going to move rapidly toward that positive magnet. What I've created here is something called potential energy. By moving this magnet away, I've created this scenario where this magnet wants to move; it wants to release this potential energy as kinetic energy, or it wants to move towards the positive magnet.

Now, when I have this nucleus think about it in terms of energy levels, we think about it in terms of energy levels because it took energy to have this electron out here at this orbital. So, there's some sort of energy associated with this energy level, and if I have multiple energy levels, I'm going to have even more energy associated with this second energy level. As I move out with each energy level away from the nucleus, the electrons that occupy these energy levels have more and more energy because they've been pulled farther and farther away from this nucleus.

Understanding energy levels
Energy Levels

If I was to talk about energy levels in terms of the first energy level or the second energy level or the third energy level, there's more energy associated with energy levels as they increase in number. So, energy is increasing as we increase the number of the energy levels.

We can also talk about energy levels in terms of the number of electrons that can occupy those levels. I'll draw my first energy level, and it's referred to as 1s, so one for the energy level, and the letter that is associated with the number is basically defining the shape of the electron cloud. It's not going to be important for our purposes to actually know what all the shapes are, but just be aware that this is what that is describing.

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