How do scientists know what an atom looks like, to make a model of one? How do they know that each molecule in the scientific charts has specific protons, neutrons, and in what orders if an atom cannot be seen? How can you prove this?
There is no simple answer to your questions. There is no "atomic microscope" which would allow one to look inside an atom and say, "Aha! There's 7 blue protons, 6 green neutrons, and 7 red electrons." The way the structure of the atom was devised was through a long series of experiments. Each one was designed to look at a specific aspect of the atom. At one time the atom was thought to be a solid ball of positive charge with electrons embedded in it. Then in 1909, Ernest Rutherford did an experiment which demonstrated that that picture was wrong and that the positive charge was centered at the center of the atom and occupied a very small volume compared to the whole atom.
Before the neutron was discovered in 1932, the nucleus was thought to have both protons and electrons in it. The number of protons was chosen to get the correct atomic weight and the number of electrons was chosen to get the correct nuclear charge. For example, in nitrogen the number of protons was 14 and the number of nuclear electrons was 7 and the number of electrons outside the nucleus was 7. There were problems with this model. One was the classical radius of the electron was 2.8 x 10e-13 cm which is of the same general order of magnitude as nuclear radii. So, how is it possible to stuff many electrons this size in a box as small as a nucleus? Other problems were that the nuclear magnetic moments were wrong, the nuclear spin predicted spectroscopic results that were not observed, and the nuclear statistics were wrong. The discovery of the neutron lead to a revision of the model leading to the current one.
In the current model, the number of electrons in the atom is determined by
gamma and x-ray spectroscopy. The number of protons in the atom is chosen
to balance the charge of the electrons in the atom. The number of neutrons
in the atom is chosen to give the correct atomic weight for the element in
question. Many additional experiments were performed to confirm the model
as finally developed and so far the results obtained are as one would
expect from the model. This agreement between the experimental results and
the predictions based on the model is what is called proof.
Submitted by Ashlee (age 9, Dallas, TX, USA)
(September 26, 1997)