How do you determine the number of protons and neutrons in an element?

[onurbeyaz]

Assume that I have a sample of an element which is electrically neutral, I want to know the number of protons, and neutrons in atoms of this element. What kind of experiment should I do? In other words, how did they know the number of protons and neutrons of elements when they were forming the periodic table

 

[e.bar.goum]

The periodic table was determined by chemistry - electrons - and atomic weights rather than proton and neutron number. http://en.wikipedia.org/wiki/History_of_the_periodic_table
You can work out atomic weights by PV=NRT and knowing whether the gas is molecular. This method isn't great though, and some atomic weights were pretty wrong.

However, these days, if you want to determine the number of protons and neutrons in atoms of a sample you can do any number of things - but proton number, Z, is easier than neutron number, in general. To find proton number, you can do virtually any kind of elastic scattering measurement, as the elastic scattering cross section is charge dependent. To work out the mass, A , you can do mass spectrometry (accelerator based or otherwise), then the number of neutrons is A - Z. But there are a few other ways you can go about it.

 

[PhysicistMike]

Assume that I have a sample of an element which is electrically neutral, I want to know the number of protons, and neutrons in atoms of this element. What kind of experiment should I do? In other words, how did they know the number of protons and neutrons of elements when they were forming the periodic table

Use a mass spectrometer. That's what they're made for. See http://en.wikipedia.org/wiki/Mass_spectrometry