Physics problem.

Chop Logic

This a pretty simple thing, I just can't figure it out, I think the units are messing me up.

Calculate the energy of the transitions from the n = 4,5,6 energy levels to the n = 3 energy level for the hydrogen atom using the Bohr formula.

So I did, E = (Rydbergs Constant)( 1/nf^2 - 1/ni^2)

But, I keep getting an answer with the units as m^-1 (using Google calculator). Since I'm solving for energy, shouldn't I be getting an answer in eV or joules? This is messing me up because next I have to convert these energies to wavelengths, and obviously I can't do that unless I've actually solved for an energy.

Thanks.

wenchkilla

Looks like the Energy is in inverse meters due to a bunch of manipulation to get the constant.

Which, luckily for you, means to find the wavelength just take the inverse of the E value

grungebox

Use planck's formula to convert back to energy.

E=hc/lambda

In your case just hc*[answer in /m]==> energy

The Cow

You're going to want to subtract the final energy level from the initial, first.

As for getting energies in m^-1, it's not impossible, it's just unusual. I'll be honest, I just found this out myself on Wikipedia and this is technically my field so

"In spectroscopy and related fields it is common to measure energy levels in units of reciprocal centimetres. These units (cm-1) are strictly speaking not energy units but units proportional to energies, with hc being the proportionality constant."

Which makes sense, getting an inverse length, since you're measuring the inverse change in wavelength. The wiki page on the Rydberg constant has a decent breakdown in the 3rd section.

Chop Logic

At what Wench said, You mean the reciprocal, right? So if I got 12 m^-1 , the wavelength would be 1/12 m?

Sorry, I'm just still kind of lost. How do I convert this number in m^-1 into an energy?

Edit: I did what Grungebox said, and now I have my answers in Joules. I guess that is what I needed to do. Thanks a lot.

Edit2: Wait, now I have all negative energies. How is that possible?

wenchkilla

Chop Logic wrote: »

At what Wench said, You mean the reciprocal, right? So if I got 12 m^-1 , the wavelength would be 1/12 m?

Sorry, I'm just still kind of lost. How do I convert this number in m^-1 into an energy?

Edit: I did what Grungebox said, and now I have my answers in Joules. I guess that is what I needed to do. Thanks a lot.

Edit2: Wait, now I have all negative energies. How is that possible?

It takes energy input into the system to go up in levels., You're going from higher energy levels to lower, meaning the atom is releasing energy.

The important part here is that the value you are getting getting is not E but deltaE. Therefore when you release energy, deltaE is negative.

Chop Logic

Cool. Thanks a lot, that makes perfect sense.

I think I got it now, but I might come back to this thread later.

Thanks again.