How do we know the atomic number of an element?

[moonman239]

Here's what I know: Scientists used mass spectrometry to figure out the number of electrons in a neutral atom of the element. We know that if an atom has a neutral charge, the number of protons = the number of electrons - therefore, the atomic number = the number of electrons.

Would anyone care to explain the process?

 

[epenguin]

Interesting historical question which others may answer better. Perhaps here http://en.wikipedia.org/wiki/History_of_mass_spectrometry will help.

This discovery of isotopes must surely have been a revelation, and a resolution of a suspicion or conundrum at the time, yet somehow it does not come over in handed-down historical legend or scientists' folklore or pantheons as one of the more shattering discoveries of the 20th century. I wonder if there is a history book that does justice to how it was seen in its time?