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Q & A: What's inside an atom and how do you know?

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Most recent answer: 03/25/2014
Considering how small atoms are, how are protons and electrons, etc counted? And how indeed do you know the structure inside an atom?
- Barbara Brown (age 61)
Bedale Yorkshire England

Hello Ms Brown,

Oh boy!  The topics about which you are asking would make up a good part of the curriculum of a course on elementary atomic physics.  We can't answer them all here but can give you some idea of how some of them are determined. 

How are protons and electrons, etc counted?

1. You measure the charge on a single electron by way of Millikan's oil drop experiment.

2. If you know the charge of a group of electrons you divide by the results of experiment #1.

3. You can count the number of clicks on a Geiger or scintillation counter.

4. You know that the hydrogen atom has zero net charge therefor the proton has a charge equal and opposite that of the electron.

5. You can determine the ratio of charge to mass by measuring the radius of curvature of a charged particle of known momentum in a magnetic field.


How do you know the structure of an atom?

Well, you don't know to start off but you make a hypothesis and then start doing experiments to test that hypothesis.  In the early days of atomic physics there was the "plum pudding" model of atoms in which there was a glob of positive stuff and the electrons were sort of distributed at random inside.  Experiments on atomic spectra showed sharp lines which could not be accounted for.  Niels Bohr then posited a new model that had a positive nuclear core and electrons in discrete orbits.   Advances in theoretical quantum mechanics then provided a decent explanation.

So it's a matter of trial and error: you make a hypothesis and then do experiments to test it.  If the results don't agree you go back to the drawing board. 






(published on 03/25/2014)

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