Question: how did scientists calculate the atomic number of each atom?

Sam Geen answered on 27 Nov 2020:

This is actually pretty hard to do! Originally the periodic table was by atomic weight (protons + neutrons), with some small corrections. The periodic table was done by properties – they noticed that elements had similar properties with elements a bit further ahead in the list of weights, which is why it’s “periodic”, which means repeating itself.

About 100 years ago people had different ideas for how atoms worked, and suggested that the nucleus has some charge equal to the electron charge of the atom to make it neutral. People then measured the spectrum of light made by atoms to understand how the electrons worked. The neutron was only discovered in 1932, when things finally fell into place with how the atomic number works.