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## AP®︎/College Physics 2

### Course: AP®︎/College Physics 2>Unit 8

Lesson 2: Atoms and electrons

# Atomic Energy Levels

In this video, David explains how an atom can absorb and emit photons with particular values of energy and how to determine the allowed values.

## Want to join the conversation?

• at around when the electron is at a -4eV energy level... I thought it could only absorb light of energies that were exactly equal to the difference in energy levels..at around the 4 minute mark .. he says there that the electron would not absorb the 5eV photo as it isn't 4eV or 6eV.. HELP
• Think of it this way, assume these energy levels are "orbits" (it really is nothing like an orbit as the electrons aren't "just" particles but we'll use this an analogy). There are only certain "orbits" that are allowed I.e. when you add energy levels to these electrons, they have to be in discrete packets that allow the jump to the next orbit or it falls back down to the current orbit... but if you send in so much energy to the electron that you pass the 0 EV state, the electron is no longer orbiting (it has reached escape velocity) and there is no discrete packet requirement.

Again, it is dangerous to think of these things as just being particles and having orbits like Newton shows as it makes understanding QM a bit harder but it does help when thinking of the Atomic Energy Levels.
• How and why electron comes back to ground state after being excited by gaining energy?
• Because of the attraction between electron in the outer shell and the proton in the nucleus.
• At , he says that the electron will only absorb a certain photon, but at , he says that the -4ev can absorb a 7ev photon. I don't get it.
• The electron can absorb photons that will make it's charge positive, but it will no longer be bound the the atom, and won't be a part of it.
For example at -10ev, it can absorb, 4eV (will move to -6eV), 6eV (will move to -4eV), 7eV (will move to -3eV), and anything above 7eV (will leave the atom)
• What happens to the electron after it leaves the atom?
• If enough energy is absorbed by the electron it leaves the parent atom making it a cation. Since atoms rarely occur in isolation this electron is taken by a neighn=bouring atom making that atom an anion.
Note: this energy to remove an electron from the atom is known as ionization enthalpy of the element and is a characteristic of nuclear pull on the electron.

Also, Beta radiation is when a neutron decays into a proton and an electron so the electron actually comes from the nucleus in that case.
• How does the electron 'jump' from one shell state to the next? It can't move there physically because it then has to move through between two shells physically which can't happen so, does it teleport?
• I have two qustions.
1] Why doesn't the electron degeneracy pressure take place within the atom and if it does, would ever be enough to expel an electron?
2] Why don't we have any nucleus that has an electron; doesn't the strong interaction force take place between the electrons?
• Degeneracy pressure does exist in an atom. It prevents electrons in the atom from occupying the same quantum state.

Electrons don't feel the strong nuclear force.
• I'm troubled by the constraint of absorption requiring an EXACT quantitative match of photon energy and transition energy. If the exactness requirement of match can be infinite, then I would expect the match to never be good enough. Even if the levels constrained to integers or a specific denominator, then there would be some associated unit like electron volt, and then what would be special about that unit of energy measurement? Or maybe God says (or whoever), "close enough"?
• The Heisenberg uncertainty principle sets a lower bound on nature's ability to tell the difference between one quantity of energy and another. Anything within that uncertainty band is "the same".
• Can an electron absorb a photon of any energy with an absolute value higher than its energy level (anything that can get it higher than 0 eV)?
• If an electron absorbs a photon with enough energy to increase its potential energy above 0eV, then the electron becomes free and is no longer bound to the atom and the atom then becomes ionized. X-rays and high energy ultraviolet light have enough energy to ionize atoms.