How do we know there is one proton in H?

[Adeonaja]

One proton in H, two in Li etc. How do we come to know about it? And same with neuteons too.

 

[sophiecentaur]

Hi. There is a very long story associated with this. It starts with the very first ideas about moving from the elements Air, Earth, Fire and Water model to the Chemical Elements. Lavoisier isolated a number of the Elements and tried to put them in an order. Read about him here and elsewhere.

Measuring the masses of nuclei was initially done by weighing chemical compounds but now we use a Mass Spectrometer (look it up) for Ions.

 

[jtbell]

You might try reading about Henry Moseley and Moseley's Law which firmly established the concept of atomic number:

https://en.wikipedia.org/wiki/Henry_Moseley
and about Ernest Rutherford who established that "the hydrogen nucleus is present in other nuclei":

https://en.wikipedia.org/wiki/Proton
After that, physicists thought for a while that atomic nuclei contained protons (which contributed the mass), and electrons (which neutralized the charge of some of the protons). The concept of the neutron as a massive neutral particle (replacing the proton+electron combination) came around 1930-1932.

https://en.wikipedia.org/wiki/Discovery_of_the_neutron
Yes, the history is complicated and messy. One has to have great respect for the physicists of the 1900-1935 period who struggled to make sense of what they observed.