Electrons excited by absorption of a photon in Mg

• HastiM
In summary, the total spin of an Mg atom remains zero after absorption or emission of a photon. The possible states for one of the electrons in the outer shell to transition to are 3p and 4p, as there is a selection rule that requires a change in the orbital quantum number by +1 or -1. However, other states with a higher energy, such as 6p, could also be allowed due to the absence of a selection rule related to the principal quantum number. f

Homework Statement

The total spin of an Mg atom is zero and remains zero after absorption or emission of a photon. To which states can one of the electrons in the outer shell get by absorption of a single photon: 4s, 5s, 3p, 4p, 3d, 4d?

2. The attempt at a solution

I am not sure which of the states are possible. My ideas so far: When a photon is absorbed by one of the two electrons in the outer shell, the electron has a higher energy. But both electrons in the outer shell are in the state 3s (one with spin up, and the other with spin down). On the other hand, all states given above represent states with higher energy, so that all states should be possible. Is that true? Or do I missing something? I guess my answer is wrong because I ignored the total spin. But we have not discussed in class how the total spin is computed and how it is related to the energy of an atom.

Any help is very appreciated!

Best wishes

Have you learned about selection rules?

HastiM
Thank you for your help and the hint!

I just read that state transitions due to a photon emission/absorption necessarily change the magnetic quantum number by +1 or -1. This would mean that in our case only the states 3p and 4p are possible. Is that correct?

I just read that state transitions due to a photon emission/absorption necessarily change the magnetic quantum number by +1 or -1. This would mean that in our case only the states 3p and 4p are possible. Is that correct?
Yes, but its related to the orbital quantum number, not the magnetic quantum number (for which there is also a selection rule, but it is irrelevant here). Note also that there is no selection rule related to the principal quantum number, so other final states, such as 6p, would also have been allowed.

HastiM
Thank you very much! You helped me a lot!