Energy level transitions

[tmh]

With quantum energy levels, to raise an electron to a higher energy level a photon with energy "exactly equal to the energy difference" must be absorbed. So can a photon raise not an electron to a higher state with "extra" energy left? If not, what would happen when it strikes?

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[MD Odyssey]

With quantum energy levels, to raise an electron to a higher energy level a photon with energy "exactly equal to the energy difference" must be absorbed. So can a photon raise not an electron to a higher state with "extra" energy left? If not, what would happen when it strikes?

First of all, in real molecules, energy transitions can get really complicated and there are often other things going on. For simple systems, like the MCAT, the rule is that the energy of the incident photon has to be equal to the difference in energy between the original state (e.g., the ground state) and the excited state of interest.

So, what happens if a photon has energy that doesn't match one of the energy levels? Sometimes, it may get absorbed as kinetic energy, which will translate into a higher temperature. Other times it may completely miss the atom entirely - at the atomic level, everything is probabilistic, so there is no law that says it will always be absorbed. Hope this helps.

Edit: One thing to add. The scenario you described, a system with energy "left over" would be incorrect for an MCAT type of question and most likely the sort of thing which someone trying to test your understanding of energy levels would probably put in a distractor. For the purposes of the MCAT, that doesn't happen, but be aware that energy level transitions can get a bit more complicated. In particular, the simple picture of electrons dropping from excited states directly back to the ground state is not always accurate in molecules, because some of the energy can go into changing the vibrational state of the molecule. This is more or less what's happening in some types of fluorescence.

 

[tmh]

Thanks a lot.

So if there is too much energy could some get transferred to kinetic and the leftover be used to raise to a new energy level? I guess this is slightly beyond the mcat but I have a hard time imagining that there could be too much.

 

[MD Odyssey]

Thanks a lot.

So if there is too much energy could some get transferred to kinetic and the leftover be used to raise to a new energy level? I guess this is slightly beyond the mcat but I have a hard time imagining that there could be too much.

No. In general, promotion of an electron from the ground state to an excited state only occurs if the photon energy is precisely equal to the transition energy.

I can understand some of your desire to think energy is "shared", but that's not really how it works. Energy really does come in little quantized packets, which is part of what makes quantum mechanics somewhat counterintuitive. Furthermore, energy level transitions within an atom are quite rigid - they are defined in fact by the quantum numbers you learned about in freshman chemistry.

Take it as an axiom that an electron is only promoted if the incident photon energy is precisely equal to the transition energy. Anything above or below will either completely miss the atom or possibly be converted to heat energy. Obviously, there is more to the story, but it's not really appropriate to talk about for the MCAT. I mention it here only to be complete.