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## AP®︎/College Physics 2

### Course: AP®︎/College Physics 2 > Unit 8

Lesson 2: Atoms and electrons- De Broglie wavelength
- Quantum Wavefunction
- Atomic Energy Levels
- Bohr model radii (derivation using physics)
- Bohr model radii
- Bohr model energy levels (derivation using physics)
- Bohr model energy levels
- Absorption and emission
- Emission spectrum of hydrogen
- Bohr's model of hydrogen
- The quantum mechanical model of the atom

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# Bohr model energy levels (derivation using physics)

Using classical physics to calculate the energy of electrons in Bohr model. Solving for energy of ground state and more generally for level n.

. Created by Jay.

## Want to join the conversation?

- How can potential energy be negative? Wouldn't that be like saying you mass is negative?

Potential energy is energy stored in an object. Either you store energy ( +ve ) or not at all ( 0 ).

So why is it negative?

Also i heard that gravitational potential energy is negative . How can 'mgh' be -ve ?

A google search told me that 0 reference point is infinity. What does that mean why is it like that?

Please Help. These questions have been plaguing me for months.(43 votes)- In mgh h is distance relative to the earth surface. So if you lower than the earth's surface the potential eergy is negative(11 votes)

- Why do we write a single "r" in the formula of P.E? I understand how the single "r" came in the formula of kinetic energy but why do we use a single "r" in Potential energy formula?(16 votes)
- Hi, nice question.

The thing is that here we use the formula for electric potential energy, i.e. the energy associated with charges in a defined system. The Formula for electric potenial = (q)(phi)(r) = (KqQ)/r.

We use (KqQ)/r^2 when we calculate force between two charges separated by distance r. This is also known as ESF.

Due to the similarity, many people (wrongly) apply the ESF for potential energy(5 votes)

- so this formula will only work for hydrogen only right?!

and what happens if there are more than just one electron in an energy level?(6 votes)- Yes. This formula will work for hydrogen and other unielecton ions like He+, Li^2+, etc.

When there are more than one electrons, then there is repulsion between those electrons due to their same negative charge. The formula then breaks down. Also note, the Bohr model is not what actually happens. The current quantum model is believed to be the most correct.(10 votes)

- What is quantized energy level?

Thanks.(2 votes)- For energy to be quantized means that is only comes in discreet amounts. It is like if I need to give you some money, I can give you 1 cent or 10 cents but I can't give you 1/2 a cent because there are no 1/2 cent coins.(14 votes)

- The energy of these electrons is calculated as though they are in a circular orbit around the nucleus. But they're not in orbit around the nucleus. So why does this work?(4 votes)
- It doesn't work. That's why the Bohr model has been replaced by the modern model of the atom. The Bohr model only worked for Hydrogen atoms, and even for hydrogen it left a lot unexplained.

The reason it works somewhat for hydrogen is that even though the electrons are not in orbits there are certain energy concepts for orbits that have more universal applicability.(3 votes)

- What does Planck's constant represent?(4 votes)
- It tells about the energy of the frequency...... ...... Whose ratio is the Planck's constant(2 votes)

- Is this the same as -1/n2X13.6 eV(1 vote)
- Yes, it is. Chemists tend to use joules an their energy unit, while physicists often use electron volts.

The formulas are the same. They are just using different units for the constant,(6 votes)

- Why do we take the absolute value for the kinetic energy but not for the potential energy? This matter is giving me all sorts of trouble understanding it deeply :((3 votes)
- Does actually Rydberg Constant has -2.17*10^-18 value or vice-versa?(3 votes)
- I'm not sure about that ether, but yes it does equal -2.17*10^-18.(1 vote)

- My book says that potential energy is equal to -Ze^2/r. Is it correct?(2 votes)
- If your book is saying -kZe^2/r, then it is right. Z stands for atomic number. In the above video we are only dealing with hydrogen atom, so, as atomic number of hydrogen is 1, the equation is just -ke^2/r. But if you are dealing with other hydrogen like ions such as He+,Li2+ etc. ,then the atomic number(number of protons) varies and you should use equation in your book.(3 votes)

## Video transcript

- If we continue with our Bohr model, the next thing we have to talk about are the different energy levels. And so we're gonna be talking
about energy in this video, and once again, there's a lot
of derivation using physics, so you can jump ahead to the next video to see what we come up with in this video, to see how it's applied. Alright, so we need to talk about energy, and first, we're going to try to find the kinetic energy of the electron, and we know that kinetic
energy is equal to: 1/2 mv squared, where "m" is the mass of the electron, and "v" is the velocity. So, if our electron is
going this way around, if it's orbiting our nucleus, so this is our electron,
the negative charge, the velocity vector, it'd
be tangent at this point. And we know that this electron
is attracted to the nucleus. We have one proton in the nucleus for a hydrogen atom, using the Bohr model, and we know, we know, that if
we're doing the Bohr model, there's a certain radius associated with where that electron is. So we know the electron is
also attracted to the nucleus. There's an electric force,
alright, so this electron is pulled to the nucleus,
this is an attractive force. This is the electric force,
this is a centripetal force, the force that's holding that electron in a circular orbit
around the nucleus here. And, once again, we talked
about the magnitude of this electric force in an earlier video, and we need it for this video, too. We're gonna use it to come up with the kinetic energy for that electron. So the electric force is
given by Coulomb's Law, the magnitude of the electric force is equal to K, which is a constant, "q1", which is, let's say
it's the charge on the proton, times "q2", charge on the electron, divided by "r squared", where "r" is the distance
between our two charges. We know that Newton's Second Law: force is equal to the mass
times the acceleration. We're talking about the electron here, so the mass of the electron times the acceleration of the electron. The electric force is a centripetal force, keeping it in circular motion, so we can say this is the
"centripetal acceleration". Alright, let's go ahead and
write down what we know. "K" is a constant, we'll
write that in here, "q1", "q1" is the charge on a proton, which we know is elemental charge, so it would be positive "e"... "q2" is the charge on the electron. The charge on the electron
is the same magnitude as the charge on the proton,
but it's a negative value. So we have negative "e", is
the charge on the electron, divided by "r squared", is equal to the mass of the electron times the centripetal acceleration. So, centripetal acceleration is equal to "v squared" over "r". So, we did this in a previous video. We're gonna do the exact
same thing we did before. We only care about the
magnitude of the electric force because we already know the direction is always going to be towards the center, and therefore, we only care... we don't care about
this negative sign here. We can also cancel one of the "r"s. So if we don't care about... if we only care about the magnitude, on the left side, we get: Ke squared over r is equal to
mv squared, on the right side. And you can see, we're
almost to what we want. Our goal was to try to find the expression for the kinetic energy,
that's 1/2 mv squared. Here, we have mv squared, so if we multiply both sides by 1/2, right, multiply both sides by 1/2, now we have an expression for the kinetic energy of the electron. So: 1/2 mv squared is equal
to the kinetic energy. So we know the kinetic energy is equal to: 1/2 Ke squared over r Alright, so we will come
back to the kinetic energy. Next, we're gonna find
the potential energy. So the potential energy of that electron. And that potential energy is given by this equation in physics. So the electrical potential energy is equal to: "K", our same "K", times "q1", so the charge of one... so we'll say, once again,
that's the charge of the proton, times the charge of the electron, divided by the distance between them. So again, it's just physics. So let's plug in what we know. This would be equal to K. "q1", again, "q1" is the
charge on the proton, so that's positive "e", and "q2" is the charge on the electron, so that's negative "e", negative "e", divided by "r". This time, we're going to
leave the negative sign in, and that's a consequence of how we define electrical potential energy. So we get: negative Ke squared over r So we define the
electrical potential energy equal to zero at infinity. And so we need to keep
this negative sign in, because it's actually important. Alright, so now we have the
electrical potential energy, and we have the kinetic energy. And to find the total energy
associated with that electron, the total energy associated
with that electron, the total energy would be equal to: so, E-total is equal
to the kinetic energy, plus the potential energy. So this would be the
electrical potential energy. So let's plug in those values. We found the kinetic energy over here, 1/2 Ke squared over r, so
we plug that into here, and then we also found the
electrical potential energy is: negative Ke squared over
r, so we plug that in, and now we can calculate the total energy. So we get some more room... The total energy is equal to: 1/2 Ke squared over r, our expression for the kinetic energy, and then, this was plus, and then we have a negative value, so we just write: minus Ke squared over r So, if you think about the math, this is just like 1/2 minus one, and so that's going to
give you negative 1/2. 1/2 - 1 = -1/2 So "negative 1/2 Ke squared
over r" is our expression for the total energy. So this is the total energy
associated with our electron. Alright, let's find the total energy when the radius is equal to r1. What we talked about in the last video. The radius of the electron
in the ground state. And r1, when we did that math, we got: 5.3 times 10 to
the negative 11 meters. And so, we're going to be
plugging that value in for this r. So we can calculate the total energy associated with that energy level. And remember, we got this r1 value, we got this r1 value, by doing some math and saying, n = 1, and plugging
that into our equation. The radius for any integer, n, is equal to n squared times r1. So when n = 1, we plugged it into here and we got our radius. So let's go ahead and plug that in. Let's do the math, actually. So, we're going to get the total energy for the first energy level, so when n = 1, it's equal
to negative 1/2 times K, which is nine times 10 to the 9th, times the elemental charge. Alright, so we just took care of K, E is the magnitude of charge
on a proton or an electron, which is equal to 1.6 times 10
to the negative 19 Coulombs, we're going to square that, and then put that over the radius, which was 5.3 times 10 to
the negative 11 meters. And to save time, I
won't do that math here, but if you do that calculation, if you do that calculation,
the energy associated with the ground state
electron of a hydrogen atom, is equal to: negative 2.17
times 10 to the negative 18 and the units would be joules. So if you took the time
to do all those units, you would get joules here. So that's the lowest energy
state, the ground state. The energy is negative,
and I'll talk more about what the negative sign
means in the next video. Alright, so we could
generalize this energy. We could say, here we did it for n = 1, but we could say that:
E at any integer "n", is equal to, then put an "r sub n" here. Let me just re-write that equation. So we could generalize this and say: the energy at any energy level is equal to negative 1/2 Ke squared, r n. Okay, so we could now take
this equation, right here, the one we talked about and actually derived in the earlier video, and plug all of this in for our "n". So we're gonna plug all of that into here. So let's get some more
room, and continue... So the energy at an energy level "n", is equal to negative 1/2
Ke squared, over, right? So we're gonna plug in
"n squared r1" here. So this would be: n squared r1 We can re-write that. This is the same thing as: negative 1/2 Ke squared over
r1 times one over n squared. So I just re-wrote this in a certain way because I know what all
of this is equal to. I know what negative 1/2 Ke
squared over r1 is equal to. We just did the math for that. Alright, so this is negative
1/2 Ke squared over r1. And so we got this number: this is the energy associated
with the first energy level. And so we can go ahead and plug that in. We can plug in this number. We can take this number and
plug it in for all of this. So that's what all of that is equal to. So, here's another way
to write our energy. So, energy is equal to: negative 2.17 times 10 to the negative 18 and then this would be: times one over n squared. So we can just put it
over n squared like that. And then we could write it
in a slightly different way. Since that's equal to E1, we could just make it
look even shorter here. We could just say that: The energy at energy level n... So: the energy at energy
level n is equal to the energy associated with the first energy
level divided by n squared. Either one of these is fine. So we could write it like this, or we could write it like
this, it doesn't really matter which one you use, but
we're gonna be using these equations, or this equation, it's really the same equation, in the next video, and
we're gonna come up with the different energies,
the different energies at different energy levels. So we're gonna change what "n" is and come up with a different energy. So energy is quantized. Now, this is really important to think about this idea of energy being quantized. And this is one reason why the Bohr model is nice to look at, because it gives us these quantized energy levels, which actually explains some things, as we'll see in later videos. So the next video, we'll
continue with energy, and we'll take these
equations we just derived, and we'll talk some more about the Bohr model of the hydrogen atom.