Main content

## Class 12 Physics (India)

### Unit 12: Lesson 2

Atoms and electrons- 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
- Heisenberg uncertainty principle
- Bohr orbits: orbital radius and orbital speed
- Bohr model: energy levels

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Bohr model energy levels

Calculating electron energy for levels n=1 to 3. Drawing a shell model diagram and an energy diagram for hydrogen, and then using the diagrams to calculate the energy required to excite an electron between different energy levels. Created by Jay.

## Want to join the conversation?

- What does the negative sign in energy term represent ?(32 votes)
- It's all in the definition.

The energy is defined as zero when the electron is an infinite distance from the nucleus. As the electron comes closer, the energy decreases, so the energy of the system becomes negative.

Even for an H atom in an excited state, the energy is negative, but it is less negative than for an atom in the ground state.(28 votes)

- What physical process do you have to do to give energy for a transition? Do you apply voltage to a material to make electrons "jump" to a different orbit? I am having a hard time visualizing this process.(17 votes)
- another common way to give energy is by using the energy of photons from light. Sort of like the photoelectric effect, but instead of having such a high energy photon that it would kick the electron out, it simply excites it to a higher energy level.(14 votes)

- I have two interrelated questions.

1 - If Bohr's model isn't correct (as stated in many of these videos), why is it that we keep referring to it?

2 - If electrons don't go around the nucleus in definite orbits, what's the current explanation regarding the electrons' movement? How can the Bohr "laws" still apply?

***(15 votes)- Bohr's Model may be wrong but it is still "correct" for many uses/calculations, it's kinda like newtons equation for gravity Einstein may have made it better, but for most cases Newton's math works out fine(3 votes)

- if electron is attracted towards nucleus, why instead of jumping around it doesn't stick to it?(8 votes)
- The Heisenberg Uncertainty Principle says that we cannot know both the position and momentum of a particle. If an electron rests on the nucleus, then its position would be highly defined and its momentum would have to be undefined. So, it cannot completely shed its kinetic energy and must move, yet it will want to minimize its total energy, resulting in the probability wave function of the orbital.(15 votes)

- What will happen if electron get more than 13.6 eV energy photon? Will it absorb or?(5 votes)
- It will escape the potential well of the nucleus and the atom becomes ionized.(10 votes)

- What exactly are electron volts? How are they defined?(9 votes)
- They're the general unit used to define energy in electrons.(0 votes)

- what is a energy level?(5 votes)
- Think of energy levels as the different floors of a very tall building. Atoms have electrons located in on each of these floors. The higher floor the electron is on, the more energy it will have because it will be further away from the center of the atom.(4 votes)

- On each energy level, there is a limit as to how many electrons can be on the level, right. Is there a pattern to the limit of electrons on each energy level? When electrons jump levels the atom produces energy, right, is that energy electricity? How does the number of electrons placed on the energy levels affect the characteristics of the material, I think it causes the material to be more unstable, is that right?(4 votes)
- I am not going to be able to answer your questions in detail here since there is a lot to cover. This starts to get into the quantum aspects of atoms but it is not too complex.

Here is a the series of Khan Academy Videos on the quantum nature of Atomic orbitals: https://www.khanacademy.org/science/physics/quantum-physics/quantum-numbers-and-orbitals/a/the-quantum-mechanical-model-of-the-atom(3 votes)

- What about atoms with more than one electron? Are they pulled away one at a time or can you pull several at once with enough energy?(6 votes)
- My textbook book uses a different equation for calculating the energy of electrons on different levels: En=-Rh(Rydberg constant)x1/n^2. What are the differences of this one and the one Jay uses in his explanation?(5 votes)
- Rydberg Constant Value,R∞ = 10973731.568508(65) m-1

Rydberg Constant Value,R∞ = 1.097 x 10^7 m^-1

**in 1/METRE**

Rydberg Constant in Joules,1Ry = 2.17**10^-18J**

*IN JOULES

I think the value in Joules is used here as we're considering Energy, so basically the same

If I'm wrong do let me know,

Hope this helps!

Onward!(1 vote)

## Video transcript

- [Voiceover] So in that last video, I showed you how to get this equation using a lot of Physics, and so it's actually
not necessary to watch the previous video, you can just start with this video if you want. And E one, we said, was
the energy associated with an electron, and the lowest energy level of hydrogen. And we're using the Bohr model. And we calculated the
value for that energy to be equal to negative 2.17 times 10 to the negative 18 joules. And let's go ahead and convert
that into electron volts, it just makes the numbers
easier to work with. So one electron volt is equal to 1.6 times 10 to the negative 19 joules. So if I take negative 2.17 times 10 to the negative 18 joules, I know that for every one
electron volts, right, one electron volt is equal to 1.6 times 10 to the negative 19 joules, and so I have a conversion factor here. And so, if I multiply these two together, the joules would cancel and give me electron volts as my units. And so if you do that math, you get negative 13.6 electron volts. So once again, that's
the energy associated with an electron, the lowest
energy level in hydrogen. And so I plug that back
into my equation here, and so I can just rewrite it, so this means the energy
at any energy level N is equal to E one, which is
negative 13.6 electron volts, and we divide that by N
squared, where N is an integer, so one, two, three, and so on. So the energy for the
first energy level, right, we already know what it is,
but let's go ahead and do it so you can see how to use this equation, is equal to negative 13.6 divided by, so we're saying the energy
where N is equal to one, so whatever number you have here, you're gonna plug in here. So this would just be
one squared, alright? Which is of course just one, and so this is negative
13.6 electron volts, so we already knew that one. Let's calculate the energy
for the second energy level, so E two, this would
just be negative 13.6, and now N is equal to two,
so this would be two squared, and when you do that math you get negative 3.4 electron volts. And then let's do one more. So the energy for the third energy level is equal to negative 13.6,
now N is equal to three, so this would be three squared, and this gives you negative
1.51 electron volts. So, we have the energies for
three different energy levels. The energy for the first energy level is equal to negative 13.6. E two is equal to negative 3.4, and E three is equal to
negative 1.51 electron volts. So energy is quantized
using the Bohr models, you can't have a value of energy
in between those energies. And also note that your
energies are negative, and so this turns out to
be the highest energy, because this is the one
that's closest to zero, so E three is the highest energy level out of the three that
we're talking about here. Alright, let's talk about the Bohr model of the hydrogen atom really fast. And so, over here on the left, alright, just to remind you, I already showed you how to get these different
radii for the Bohr model, so this isn't drawn perfectly to scale. But if we assume that we have
a positively charged nucleus, which I just marked in red here, so there's our positively charged nucleus. We know the electron orbits
the nucleus in the Bohr model. So I'm gonna draw an electron here, so again, not drawn to scale, orbiting the nucleus. So the positively charged nucleus attracts the negatively charged electron. And I'm saying that electron
is orbiting at R one, so that's this first radius right here. So R one is when N is equal to one, and we just calculated that energy. When N is equal to one, that was negative 13.6 electron volts, that's the energy associated
with that electron as it orbits the nucleus. And so if we go over here on the right, and we say this top line here represents energy is equal to zero, then this would be negative
13.6 electron volts. So none of this is drawn
perfectly to scale, but this is just to give you
an idea about what's happening. So this is when N is equal to one, the electron is at a distance
R one away from the nucleus, we're talking about
the first energy level, and there's an energy of negative 13.6 electron volts associated with that electron. Alright, let's say the
electron was located a distance R two from the nucleus. Alright, so that's N is equal to two, and we just calculated that energy is equal to negative 3.4
electron volts, alright? And then let's say the
electron was at R three from the nucleus, that's
when N is equal to three, and once again we just
calculated that energy to be equal to negative
1.51 electron volts. And so it's useful to compare
these two diagrams together, because we understand this
concept of energy much better. For example, let's say we
wanted to promote the electron that I drew, so this electron
right here I just marked. Let's say we wanted to
promote that electron from the lower energy level
to a higher energy level. So let's say we wanted
to add enough energy to cause that electron to go
from the first energy level to the second energy level, so that electron is jumping up here to the second energy level. We would have to give that
electron this much energy, so the difference in energy
between our two energy levels, so the difference between
these two numbers. And if you're thinking about
just in terms of magnitude, alright, 13.6 minus 3.4, alright? So this is a magnitude
of 10.2 electron volts. And so if you gave that
electron 10.2 electron volts of energy, right, you
could cause that electron to jump from the first energy level all the way here to the
second energy level. But you would have to provide
the exact right amounts of energy in order to get it to do that. Alright, let's say you
wanted to cause the electron to jump, let's say, from
the first energy level all the way to the third
energy level, alright? So from the first energy level
to the third energy level. So that would be, here's our electron in the first energy level, let's say we wanted to cause it to jump all the way up to here, alright? So once again, you would have to provide enough energy in order to do that. So, just thinking about
the magnitudes, right? This was negative 1.51,
this was negative 13.6, if we just take 13.6 minus 1.51, alright, we would get how much
energy we need to put in in order to cause that transition, so this would be 12.09 electron volts. And so if you gave an
electron 12.09 electron volts, you could promote it to
a higher energy level. And then finally, the last situation, let's think about taking the electron, let me go ahead and draw
it in here once more, in the first energy level. And let's say you provide
it with enough energy to take it an infinite
distance away from the nucleus, so again, not drawn to scale. So let's say we're at an infinite distance away from the nucleus. If the electron is infinitely
away from the nucleus, it feels no attractive pull. So there's no force,
there's no attractive force, we talked about Coulomb's Law earlier. So this is when R is
equal to infinity here, and if there's no attractive force, there's no potential energy. So the way we define potential energy, electrical potential
energy, it's equal to zero when R is equal to infinity. So the electrical potential
energy is equal to zero, and if it's not moving, then it's kinetic energy is equal to zero, and therefore it's total
energy is equal to zero. So this is what the diagram
over here on the right means. So when E is equal to zero, we're talking about the electron being an infinite distance
away from the nucleus, so we can say N is equal
to infinity, alright? R is equal to infinity,
and if it's not moving, it has a total energy equal to zero. So we've taken the
electron completely away from the nucleus, we
have ionized it, alright? So we've gone from a neutral hydrogen atom to the hydrogen ion, so
this turns it into H plus, so we're going from H to H plus. And that amount of energy, let me use a different
color here, so obviously it requires a lot of
energy in order to do that, so that would be going
from an electron here to an electron here, so
what is the magnitude of that energy difference? That's 13.6 electron volts. So it takes 13.6 electron
volts to take an electron away from the attractive
pull in the nucleus, and to turn it into an ion. And this number, 13.6 electron volts, corresponds to the ionization
energy for hydrogen. And so the Bohr model accurately predicts the ionization energy for hydrogen, and that's one of the
reasons why it's useful to study it and to think about these different energy levels. So not only are the
radii quantized, alright, just going back over here, not only are these radii quantized, but the energy levels are, too.