## Rydberg's Equation

$E=hv$

Raashi Chaudhari 2D
Posts: 40
Joined: Wed Sep 30, 2020 9:31 pm

### Rydberg's Equation

Hi! Sorry this might be in the wrong category, but I couldn't find one regarding Rydberg's Equation.
Anyways I have some confusion after a UA step up session that I hope can be clarified!

In Rydberg's equation, $\frac{c}{\lambda R }= ((\frac{1}{n_{1}^{2}})-(\frac{1}{n_{2}^{2}}))$ I've noticed that people switch the n1 and n2 in the equation. In which scenarios do we do final-intial and which do we do initial-final? Is this the correct equation??

Abril Guanes 3D
Posts: 55
Joined: Wed Sep 30, 2020 9:43 pm

### Re: Rydberg's Equation

Hello!
For the most part, it's Final - Initial. I can't think of a situation where we would use Initial - Final. As for the equation, I've heard that you switch N1 and N2, dependent upon whether you're going to get a negative or positive. So, if the problem mentions absorption you would want a positive answer, but if the problem mentions emission, you would want a negative answer. I don't tend to use this equation, and rather use E(n) = -hR/(n)^2, for each of the energy levels and just subtract the final energy from the initial energy. Hope this helps!

Becca Nelson 1J
Posts: 48
Joined: Wed Sep 30, 2020 9:52 pm

### Re: Rydberg's Equation

It is final - initial. In the case of energy being emitted it will be positive and absorbed will be negative. While your equation is correct, I find it easier to use v=R(n1^-2 - n2^-2). Hope this helps!

Tanya Nguyen 1E
Posts: 40
Joined: Wed Sep 30, 2020 9:33 pm

### Re: Rydberg's Equation

It would usually be final - initial, but I heard that some people switch it up. Just be sure that if you do, remember where things are and possibly have to switch the sign of your answer. There is also another way to write the equation and it is the equation given on the equation sheet linked on the chemistry website: v = R[1/n(final)^2 - 1/n(initial)^2]

Raashi Chaudhari 2D
Posts: 40
Joined: Wed Sep 30, 2020 9:31 pm

### Re: Rydberg's Equation

Abril Guanes 3D wrote:Hello!
For the most part, it's Final - Initial. I can't think of a situation where we would use Initial - Final. As for the equation, I've heard that you switch N1 and N2, dependent upon whether you're going to get a negative or positive. So, if the problem mentions absorption you would want a positive answer, but if the problem mentions emission, you would want a negative answer. I don't tend to use this equation, and rather use E(n) = -hR/(n)^2, for each of the energy levels and just subtract the final energy from the initial energy. Hope this helps!

Hi! Thank you that was helpful!!
I usually prefer to use the equation you put down as well, but I was confused whether I would get the same answer by using the one I listed.
For the equation you listed, you said we can solve for our E final, then our E initial and subtract the two to find the change in energy; but, if a question asks to find the wavelength, can we set that change of E=hc/lambda? Then solve for lambda?

Abril Guanes 3D
Posts: 55
Joined: Wed Sep 30, 2020 9:43 pm

### Re: Rydberg's Equation

Raashi Chaudhari 2D wrote:
Abril Guanes 3D wrote:Hello!
For the most part, it's Final - Initial. I can't think of a situation where we would use Initial - Final. As for the equation, I've heard that you switch N1 and N2, dependent upon whether you're going to get a negative or positive. So, if the problem mentions absorption you would want a positive answer, but if the problem mentions emission, you would want a negative answer. I don't tend to use this equation, and rather use E(n) = -hR/(n)^2, for each of the energy levels and just subtract the final energy from the initial energy. Hope this helps!

Hi! Thank you that was helpful!!
I usually prefer to use the equation you put down as well, but I was confused whether I would get the same answer by using the one I listed.
For the equation you listed, you said we can solve for our E final, then our E initial and subtract the two to find the change in energy; but, if a question asks to find the wavelength, can we set that change of E=hc/lambda? Then solve for lambda?

Hey again!
So when you solve using the equation (solving for the energy of the initial and the energy of the final state and subtracting it) that usually gives you a negative energy, which represent the CHANGE in energy of the ELECTRON after going from the initial state to the final state. Because the electrons are going from a high energy state to a lower energy state (the ground level), the electrons are emitting energy. To find the energy of the photon being emitted you would simply multiple the change in electron of the electron by -1, giving you a positive number. THEN, you can use the positive energy of the photon in the equation you listed. Sorry for the lengthy explanation, but I think it's really important to grasp what each of the equations give you as an answer. Happy studying!

Nina Tartibi 1F
Posts: 37
Joined: Wed Sep 30, 2020 9:58 pm

### Re: Rydberg's Equation

Hey again!
So when you solve using the equation (solving for the energy of the initial and the energy of the final state and subtracting it) that usually gives you a negative energy, which represent the CHANGE in energy of the ELECTRON after going from the initial state to the final state. Because the electrons are going from a high energy state to a lower energy state (the ground level), the electrons are emitting energy. To find the energy of the photon being emitted you would simply multiple the change in electron of the electron by -1, giving you a positive number. THEN, you can use the positive energy of the photon in the equation you listed. Sorry for the lengthy explanation, but I think it's really important to grasp what each of the equations give you as an answer. Happy studying![/quote]

So when do you know to negate the negative sign again? Is it when you get a negative delta energy because of absorption and then to find Ephoton you'd need to make that number positive?

Summer_Corona 3F
Posts: 35
Joined: Wed Sep 30, 2020 10:06 pm

### Re: Rydberg's Equation

Nina Tartibi 1F wrote:Hey again!
So when you solve using the equation (solving for the energy of the initial and the energy of the final state and subtracting it) that usually gives you a negative energy, which represent the CHANGE in energy of the ELECTRON after going from the initial state to the final state. Because the electrons are going from a high energy state to a lower energy state (the ground level), the electrons are emitting energy. To find the energy of the photon being emitted you would simply multiple the change in electron of the electron by -1, giving you a positive number. THEN, you can use the positive energy of the photon in the equation you listed. Sorry for the lengthy explanation, but I think it's really important to grasp what each of the equations give you as an answer. Happy studying!

So when do you know to negate the negative sign again? Is it when you get a negative delta energy because of absorption and then to find Ephoton you'd need to make that number positive?[/quote]

Yes, that's right anytime you get a neg change in energy you would change the sign, since the energy being lost by the electron is being released as EM, radiation, and use it to continue solving the problem. However, I believe you're getting a neg change in energy due to emission not absorption as you're decreasing in energy levels.

Raashi Chaudhari 2D
Posts: 40
Joined: Wed Sep 30, 2020 9:31 pm

### Re: Rydberg's Equation

Summer_Corona 3F wrote:
Nina Tartibi 1F wrote:Hey again!
So when you solve using the equation (solving for the energy of the initial and the energy of the final state and subtracting it) that usually gives you a negative energy, which represent the CHANGE in energy of the ELECTRON after going from the initial state to the final state. Because the electrons are going from a high energy state to a lower energy state (the ground level), the electrons are emitting energy. To find the energy of the photon being emitted you would simply multiple the change in electron of the electron by -1, giving you a positive number. THEN, you can use the positive energy of the photon in the equation you listed. Sorry for the lengthy explanation, but I think it's really important to grasp what each of the equations give you as an answer. Happy studying!

So when do you know to negate the negative sign again? Is it when you get a negative delta energy because of absorption and then to find Ephoton you'd need to make that number positive?

Yes, that's right anytime you get a neg change in energy you would change the sign, since the energy being lost by the electron is being released as EM, radiation, and use it to continue solving the problem. However, I believe you're getting a neg change in energy due to emission not absorption as you're decreasing in energy levels.[/quote]

So, to clarify, we get a negative Delta E when the question is asking about an emission and a make it positive if its asking for an absorption?

Danielle Goldwirth 3F
Posts: 49
Joined: Wed Sep 30, 2020 10:07 pm

### Re: Rydberg's Equation

When a photon emits electromagnetic radiation, the energy decreases, so yes, delta E is negative.
However, when solving for the frequency/wavelength of light emitted by a Hydrogen atom, we make this energy value positive because the energy of the photon is equal and opposite.