Cartoon Model Of Energy Levels In Different Shells

So that's it? Atoms really look like little solar systems with electrons making quantum jumps between special orbits?
 

Well, not quite. The idea of an electron actually flying around in little circles turned out to have lots of problems, and physicists were eventually forced to discard that model.
 

But we just finished talking about how well that worked! Why do we have to throw the whole thing away?
 

We're not going to start from scratch. The concept of "special orbits" was extremely useful, it's just the orbits themselves that we're not going to use anymore. Instead, we're going to think about electrons being in special energy levels. We just use this rule:

Oh, that's easy enough. But why bother? Why not just call them orbits?
 

Well, first of all, some orbits have the same energy as other orbits, so sometimes changing orbits wouldn't emit radiation. Also, it turns out that electrons don't really move in little circular orbits. We can take a little detour to see how the Schrödinger Atom (below) more accurately depicts what is happening inside atoms.
 

Actually, thinking about energy levels makes more sense, anyway, because if the energy goes down the extra energy has to go somewhere, so it comes out as electromagnetic radiation.

Yeah, and in order for the energy to go up it has to come from somewhere, so it takes some incoming radiation!

This next applet shows the Bohr model along with a diagram showing the energy level. This "energy level" picture of an atom is so useful that most physicists prefer it over the orbital picture.
 

Hold on. Earlier we were saying that when an electron changes its speed or direction, it gives off electromagnetic radiation. Now we're saying that when an electron changes its orbit (or "energy level") it gives off electromagnetic radiation. Which is it?
 

You're changing your story on us! Are you making this up as you go along or what?
 

Change in velocity was a classical idea, but the quantum physicists realized the important part is that the energy of the electron changes, and electromagnetic radiation makes up the difference. If the energy goes down, the extra energy appears as a photon. And for the electron to get more energy, it needs to absorb a photon. Now let's look at how this theory neatly explains spectral lines...
 

So what's wrong with the idea of electrons going around in orbits?
 

A physicist named Erwin Schrödinger showed that electrons are really waves that...
 

Hold it! What do you mean electrons are waves?! I thought they were particles!
 

This is where quantum physics gets truly bizarre. If you perform an experiment to see where a particle is, then you find something particle-like. But otherwise it's a wave that carries information about where the electron probably is. The famous Two-Slit Experiment is another way of looking at the dual wave/particle nature of electrons.
 

What do you mean when you say the electron "probably is" somewhere? Isn't the electron in some definite place?
 

Well, no. Until you check where it is, it's really just a wave. Not only that, but Schrödinger showed that these electrons don't even move. The waves are stationary. Each time you check where an electron is you will find it in a different place, but that doesn't mean it's moving in between checks. For some energy levels, if you check position enough times you may see an "orbit-like" pattern, but don't be fooled into thinking that electrons are actually moving around in little circles.
 

So where is an electron when you're not looking? Doesn't it have to be somewhere?
 

That's the bizarre part: an electron isn't in any particular place when you aren't looking. Fortunately, for most physics it doesn't really matter where it actually is, we only care about how much energy it has.
 

Oh! And that's why orbits are useful! They may be misleading about where the electron is, but they do tell us how much energy it has.
 

We call this the Energy Level of the electron. Because the idea of orbits is so misleading, physicists started using a picture of the atom which just showed energy levels as relative heights.
 

And we call this the "Schrödinger Model," of course. So in this applet, the picture to the left of the Bohr Model shows the energy level of the electron, and the picture to the right shows the areas around the nucleus where the electron will probably be found.
 

You can interact with the Bohr hydrogen atom in this applet just like the last one, except now you will see the changes in the Schrödinger model as well.