# AP Physics 2 : Electron Energy Levels

## Example Questions

### Example Question #1 : Electron Energy Levels

Suppose that an electron within a hydrogen atom moves from the fourth energy level to the second energy level. What is the wavelength of the photon emitted during this process?

Explanation:

To answer this question, we'll need to utilize the equation that specifies the energy level of electrons within a hydrogen atom.

Where  is equal to the electron energy level within the hydrogen atom. Also notice that this equation has a negative sign. This is because in its ground state, an electron is closest to the positively charged nucleus and thus has the lowest energy. As the energy level increases, the electron moves further and further away from the nucleus, thus gaining increasing energy. At an infinitely far away energy level, the electron will have a maximum energy value of zero. To find the difference between the second and fourth energy levels, we'll simply use the above equation for different values of .

The negative sign for the change in energy just means that energy is being released in this process. We can drop the negative because we know that energy is being released.

Now that we've found how much energy is contained in the released photon, we'll need to calculate its wavelength.

### Example Question #2 : Electron Energy Levels

An electron collides with an atom, exciting an electron in the atom from it's ground state . The initial velocity of the incoming electron is  and after the collision it has a velocity of . What is the energy of the excited electron in the atom after the collision in electron-volts?

Explanation:

The incoming electron will lose kinetic energy during the collision, transfering this energy to the potential energy of the bound electron in the atom. Conservation of energy can be used to solve this problem. The general statement that energy is conserved is

where  is the kinetic energy and  is the potential energy. The incoming electron has kinetic energy and no potential energy. We are defining the initial state of the bound electron to be at  so the total initial potential energy of the system is zero.

The incoming electron will still have kinetic energy after the collision but the bound electron will not since it is not a free electron. This means that

where

plugging this in -

is the mass of the electron. Plugging everything in and converting to  gives

### Example Question #3 : Electron Energy Levels

Calculate the energy released as a photon when an electron falls from the  energy level to the  energy level.

Explanation:

During a energy level change in a hydrogen atom, the amount of energy either lost of gained is given by the following equation with respect to the initial and final energy levels shown below.

Recall that whenever electrons drop from higher energy levels to lower ones, energy can be released in the form of a photon. To obtain the amount of energy released, we mst take the difference in energy of the electrons at the particular energy levels:

It is important to note that the negative energy difference corresponds to how much energy the photon is "taking away" as it leaves. Therefore, the photon leaves the atom with  of energy.

### Example Question #4 : Electron Energy Levels

An electron in a hydrogen atom falls from the level to the level. What is the energy of the photon emitted?

None of these

Explanation:

Using

Plugging in values:

This will be the change in energy of the electron, which is the negative of the energy of the photon released.

Thus, the energy of the photon is

### Example Question #5 : Electron Energy Levels

How much energy would it take to raise an electron from the to the energy level of a hydrogen atom?

None of these

Explanation:

Using the formula for the energy of an electron in a hydrogen atom's nth energy level:

Plug in  and  then find the difference:

Convert electronvolts to Joules:

### Example Question #6 : Electron Energy Levels

One mole of hydrogen atoms have electrons drop from the  energy level to the  energy level. Determine the energy released.

None of these

Explanation:

Using the following equation for the energy of an electron in Joules:

And

Combining equations and plugging in values:

would be released

### Example Question #7 : Electron Energy Levels

What is the difference in energy for a hydrogen atom with its electron in the ground state and a hydrogen atom with its electron in the  state?

Explanation:

For this question, we need to compare the difference in energy levels of hydrogen atoms with electrons in different orbitals.

First, we will need to use the equation that describes the energy of an electron in a hydrogen atom.

In the above expression,  represents the orbital in which the electron resides.

First, let's see what the electron energy level is in the ground state, which corresponds to .

Next, let's do the same thing for the  orbital.

Next, we can find the difference in the energy values.