Electron Shells and Bohr Model
Name: Dale H.
One of my 10th grade science students asked a questions regard the Bohr model of
the atom which I could not answer. She asked "How did Bohr know that the first electron shell
held only 2 electrons? then, how did he know the second held only 8?"
Bohr's initial contribution was the explanation of the electronic spectrum of the hydrogen
atom, which contains only a single electron. The extension of idea to atoms containing more
than a single electron depended upon a rapid succession of both experimental and theoretical
results. For example, Franck and Hertz (between 1914 and 1920) measured the energy of electrons
scattered from various targets. They found two things:
1. The scattered electrons usually had much less energy than the incident electrons, but
that change in energy occurred in steps.
2. Sometimes more electrons were produced (current) than were incident upon the target.
We now know that those observations corresponded to the incident electrons exciting electrons
in the target to higher energy levels, and to the ionization of the target electrons,
Concurrently, in the area of atomic theory Schroedinger (and others) were developing what
we now refer to as quantum mechanics. First, for atoms containing a single electron, and
later extended to multi-electron atoms. The result of the theory of multi-electron atoms
was that stable electronic states corresponded to appearance of certain integers (we now
refer to them
as quantum numbers). The results of the theory for atoms required the following constraints on
the values of those quantum numbers: There were 3 quantum numbers (n, l, s). Pauli showed that
no two electrons in an atom could have the same set of (n, l, s). The details of this history
is too large to present in detail here, but you will find it explained very well in the
classic text"General Chemistry" by Linus Pauling Chapter 5.
Bohr was about 1912 with all this, I think. At that time, he would have known how many
electrons are in a shell by spectroscopic measurements. New energy levels would show up,
new emission lines, for the elements as one went from hydrogen on up. This would first be
put into ad-hoc rules (by the spectroscopists) but then given a mathematical basic by
quantum mechanics maybe 15 years later.
I suggest you read or recommend the student read The Ascent of Man, by Jacob Bronowski. There
are some good history chapters in it, of these early days. There was a PBS TV series by him
(the book came from the TV), it may still be in libraries. There are many such books, but I
have always liked this one. It is fine for 10th grade.
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Update: June 2012