1940 was in an era that was a golden age in atomic physics.
The planetary model we use today had just recently been
discovered in experiment by a scientist named Ernest
Rutherford who had been working on an experiment involving
shooting electrons at a gold foil and recording the scattering
effect. He was shocked to determine that the old models were
wrong and that atoms in fact consisted of a nucleus being
orbited by small electrons. Previous to his experiment the
thought was that an atom was one solid thing with a balanced
charge, however when he did the experiment shown below he
discovered that the atom was in fact largely empty, a large
central body with smaller orbiting components circling it. It
was quickly shown that the atom is in fact a center nucleus of
protons and neutrons orbited by smaller electrons. Image
thanks to_https://www.youtube.com/watch?v=Qi_EMo9nGOw.
Shortly afterwards the theory was solidified by Neils
Bohr's discovery that electrons move in discrete energies
(or "n" levels) which translate to set discrete orbital
levels from the nucleus. This provided the last needed
information to give us confidence in the new planetary model
because it was a way of accounting for orbital decay and
negating it. Image thanks to http://thehistoryoftheatom.weebly.com/niels-bohr.html
Over the next 75 years enormous scientific progress was
made in this field. Now we know more than we ever using this
new model. However, this new model opened the door to new
questions, it started becoming apparent that all electrons
are completely uniform in density, uniform in appearance,
and due to the Heisenberg Uncertainty Principle, we cannot
exactly prove where any one atom is at any moment. Image
below thanks to https://electronics.stackexchange.com/questions/332862/how-the-resistor-limits-the-current-across-the-circuit-when-only-one-part-of-the
showing how electricity works, which is only usable with
uniform electrons, if electrons weren't uniform in every
detectable way electricity would not be easily usable and
predictable at the outlet, as it is in reality.
