How did they figure out how many electrons and how many protons and how many neutrons each element has?

Question

Clearly they knew these values in the early 20th century, even late 19th. How did they figure it out??

Answer 1

The most basic work was done before Mendeleev came up with the periodic table, because he used the basic data to do so. At that time, the neutron had not been discovered, so the idea of atomic number was not well understood -- it was just the count of the element's position in the table. The earlier data was based on the combining weights of various elements: how many grams of sulfur combine with iron to make iron sulfide, for example. The discovery of x-rays shed new light on the whole subject (sorry about the pun), because the wavelengths of absorbed x-rays gave a direct indication of atomic number, and beautifully confirmed Mendeleev's work. The difference between atomic number and atomic weight was not explained until Chadwick discovered the neutron, and that pretty well settled the numerical issues. Explanation of the details of atomic bonds had to await the discovery of the quantum theory by Schrodinger and others.

Answer 2

It began with Thomson and Millikan's work with electrons and the charge/mass of electrons in 1890's. Then Rutherford and his students determined that the atoms had a positively charged nucleus. From there Moseley determined the number of protons in the atoms of each element. Using a mass-spectrometer (fancy cathode ray tube) Chadwick realized that the nucleus was heavier than expected and predicted the neutrons.

All of these discoveries were made with rather simple equipment but also with intelligent interpretation of the information gathered by that equipment.

Answer 3

Here is Wikipedia's entry. It's not very clear, though. I will look more later. I've seen better:
http://en.wikipedia.org/wiki/Atomic_theory