Atomic Shells

I was wondering how we know that every atom has 7 shells that electrons can exist in even if that atom could not hold that many electrons. For example, Hydrogen can only hold 2 elections max, this only requires the first electron shell. And yet, both my science teacher and the textbook states that it has seven shells. So my question is: How do we know? I can not think of any way to measure the existence of these six other shells, how are they there? and how do we know? I mean, the reason the electron is attracted to the proton is due to its opposite force, and one proton attracts one electron in every atom. So, wouldn't the shell be directly related to the number of protons, and thus the attraction to electrons? That's just my thinking. So please if you can, tell me How we know that every atom has seven electron shells. Thank you and good night (I'm writing this at 11:30 P.M.)
- D.T. (age 15)
Maumelle Arkansas

The "shells" aren't really something that's sitting there. They're just a description of collections of possible states for the electron waves, with each shell being a collection of states with the same or nearly the same energy. The wave equation for the electrons isn't all that hard to solve, so you find that even for simple hydrogen there is an infinite collection of different possible energy levels for a single bound electron. In practice, an atom can be prepared with the electron in any of the low-lying energy levels, using light to manipulate it. Any state except the lowest-energy ground state doesn't last very long, since the electron falls to lower energy giving off the extra energy in the form of light. The higher energy levels, with the electrons almost unbound, are very closely spaced in energy and very short-lived. I guess the number "7" that your teacher gave is based on the difficulty of making measurements of the higher energy levels. There's nothing fundamental about it.

Mike W.

(published on 04/05/2017)