Bohr's Model: Atomic Emission & Absorption

Electrons can move up and down to different orbitals or shells based on absorbing or releasing of energy. 

Bohr's Model of the Atom

Concept: Understanding the Atom

2m
Video Transcript

In this new video, we're going to continue with the discussion of the atomic model. We're going to say an atom is composed of three subatomic particles. On our right side, we have a picture of our atom. Remember, in the center of an atom is where we find the nucleus. This right here represents our nucleus. Now it's not drawn exactly to scale. Remember the nucleus contains the majority of the mass of an atom, but the nucleus itself is very small compared to the rest of the atom.
We're going to say that the nucleus, being in the center, contains two of the three subatomic particles. It contains the protons and the neutrons. We're going to say spinning around our nucleus we find the third subatomic particle, our electrons. Remember that the protons are positively charged, remember that the electrons are negatively charged and remember that the neutrons have no charge, so non-charged subatomic particles.
Because the nucleus is composed of protons which are positive and neutrons which are neutral, the nucleus itself is positively charged. We're going to say that, we're going to cancel out those positive charges by the electrons that spin around the nucleus, so that's why an atom is a neutral species. 

Bohr's Model tries to explain what happens to the electron when it absorbs or emits energy. 

Concept: Absorption vs. Emission

5m
Video Transcript

We've learned all these things before. Here's what we come up with new understandings of the atom and how the electrons work within it. Now, we're going to say according to Bohr's model, it helps explain what happens when electron absorbed or released energy within a hydrogen atom.
What we're going to say here is, this blue right here represents our nucleus, where we find our protons and our neutrons. And here we’re going to say that this first circle here represents our first shell. In our first shell, we have an electron. That electron is going to absorb light energy from a photon. When that electron absorbs this energy, it becomes excited.
So that's what we're going to put down here. We're going to say, when it absorbs enough energy, it becomes excited. Once it becomes excited, it's able to jump from the first shell or whatever shell it's into a higher leveled shell. The electron absorbs energy and it jumps up to here, so that's why we find it in the second shell.
The thing is, electrons that absorb energy, they can't hold on to the energy forever. Eventually, they're going to release this energy slowly back from where it came. So the electron is slowly going to release this energy and this energy is going to be emitted as heat or light and when it's releasing this energy back from where it came, it's going to come and return back to what we call its ground-state level. Ground state level just means that it returns to the level it was at before it absorbed that energy.
What we should see to the right of this -- so we've already talked about what happens when it absorbs energy—it jumps up to a higher level. When it releases energy it falls back down to its previous level.
Here on the right side, we have a diagram. Now, this diagram basically shows us how much energy is required when we go from one shell to the next shell. As you can see here, this represents shell number one, shell number two, three, four, infinity, and we're going to say that in this model, distance equals energy.
So you can see that going from shell one to shell two is the biggest difference, the biggest distance involved. Because of that, we're going to say going from one to two or two to one involves the most amount of energy, because we said that the distance equals energy. You’re also going to notice as we go from two to three, the distance gets a lot smaller, so we're going to say going between two and three requires less energy. As we go from three to four, it gets even smaller.
You can see in the pattern if we're trying to go from level four to five, it'd be even smaller still and going from six to seven—it will be almost like doing nothing. You'd be able to just step over into the next shell. So we're going to say the higher up you go in shell number, the smaller the distance is and the less energy is required of us to go from one to the other.
Just remember this, the lowered number shells, there's bigger difference in distance between them and because of that, it takes more energy. So it takes more energy to go from one to two than it does to go from three to four.
Just remember when we talk about absorption, we're having the electron go from a certain level up to a higher numbered level. When it releases or what we say emits this absorbed energy, it's going to fall back down. It's going to go from a larger number, back down to a smaller number.
Just remember these concepts, what's the difference between absorption and emission and remember distance equals energy in terms of the electron going from one shell to the next shell.

In absorption, an electron gains energy and becomes excited. In this excited state, the electron moves to higher energy level. 

In emission, the electron releases its excess energy to go down to a lower energy level.

Example: Calculate the energy of the 4th electron found in the n = 2 state of the boron atom 

6m

Problem: Which of the following transitions (in a hydrogen atom) represents emission of the longest wavelength? a) n = 4 to n = 2 b) n = 3 to n= 4 c) n = 1 to n = 2 d) n = 6 to n = 5 e) n = 2 to n = 5

2m

Problem: Which of the following transitions represents absorption of a photon with the largest energy? a) n = 3 to n = 1 b) n = 2 to n = 4 c) n = 1 to n = 2 d) n = 6 to n = 3 e) n = 1 to n = 4

2m

Atomic Emission

Concept: Understanding Absorption & Emission

2m
Video Transcript

In this new video, we're going to take a look at atomic emission. Now, we'd say that when electrons absorbs enough energy, it's going to go from a smaller numbered shell and it's going to use that energy to push itself up to a higher numbered shell. So example, it's going to go from shell number two up to shell number four, and we use n to represent that shell number.
Now, in the opposite way, that electron eventually can release or emit the energy that it absorbed and when it does this, it goes from the higher numbered shell to the lowered numbered shell. It goes back to where it came.
Remember the difference between absorption where we're taking energy in and using that energy to jump up to a higher level. Releasing or emitting means that we're releasing that energy that we absorb. When we release that energy, we fall back down to what we call our ground state, our normal energy level. 

Concept: Different Emission Series

2m
Video Transcript

Now, we're going to say, depending on which level we rest back on, we have different types of emission series. Now, we're going to say, if the electron goes from a higher numbered shell to the first shell, it's referred to as a Lyman series. Here we have the sign infinity, meaning that we could start from any shell, shell two to shell seven, it doesn't matter, as long as we wind up going back down to shell number one, it's going to be a Lyman series.
We're going to say if the electron goes from a higher numbered shell to the second shell, it's no longer called the Lyman series—it's going to be called a Balmer series. Again, any number higher than two, three to seven, if we start at any one of those levels and come back down and rest at level two, shell two, then it's a Balmer series.
Then, we're going to say if the electron goes from a higher numbered shell, if it starts from the third shell -- higher numbered shell then goes down to the third shell, then it’s going to be called a Paschen Series. So we're going to start out at levels higher than three, so from four to seven. You start at shell six you can go down to three.
Just remember, remember the difference between absorption versus emission. Remember, we have different types of emission series, depending on what level we fall back down to, whether it will be level one shell one, shell two or shell three. They each have different names to them and they all have different energies. Because remember, we're releasing energy, depending on where you fall, you're releasing different quantities of energy. 

We know that emission is the releasing of energy, but different energies are released depending on which shell the electron falls. These different energies appear in different places on the electromagnetic spectrum. 

Example: What is the wavelength of a photon (in nanometers) absorbed during a transition from n = 4 to n = 2 state in the hydrogen atom?

6m

Problem: Classify each of the following transitions as either a Lyman, Balmer or Paschen series. a) n = 3 to n = 1 b) n = 6 to n = 1 c) n = 3 to n = 2 d) n = 6 to n = 3 e) n = 4 to n = 2

2m

Bohr's Model: Atomic Emission & Absorption Additional Practice Problems