Re: How do we know # of protons and neutrons in an atom if we can't see it?

This is a really good question because the answer is not exactly what you might expect. There is no direct way to "see" subatomic particles (protons, neutrons, and electrons) in an atom. The way atoms of an element are "seen" is through experiment and scientific deduction not through direct measurement.

First, the general structure of an atom had to be determined, an equation if you like, that describes what particles make up an atom and how the number of these particles are related in any atom. This evolved over time but the highlights are as follows: J.J. Thompson developed the first reasonable model of the atom in 1904 (he discovered the electron in 1897 and the proton in 1907). His model said the atom consists of a sphere of positive charge within which was buried negatively charged electrons (also known as the "plum pudding" model of the atom). Ernest Rutherford proposed a different model of the atom in 1911 which said that most of the mass of the atom, and all its positive charge, resides in a very small dense centrally located region called the "nucleus". Additionally, this model said that most of the total volume of the atom is empty space within which the negatively charged electrons move around the nucleus. This model however contained a flaw because it did not take into account neutrons which were not discovered until 1932 by James Chadwick (although Rutherford had actually proposed the existence of the neutron). That gave us the basic atomic structure that we know today, but what about the relationship between the particles? Well, that began to fall into place around 1913 when Henry Moseley began studies using X-ray spectroscopy and eventually worked out that the number of protons in the nucleus of an atom could be calculated from what is called the K-line of an X-ray spectrum. This lead to the discovery that atomic number gives the number of protons. It had also been discovered that the number of protons and number of electrons are always the same (in a neutrally charged atom). Now, since the nucleus of an atom is composed of neutrons and protons and contains nearly all the mass of the atom (the mass of the proton is 1836 times that of the electron), the number of neutrons can easily be calculated from the atomic mass and the atomic number, which leaves us with the following relationships:

Number of Protons = Atomic Number
Number of Electrons = Number of Protons = Atomic Number
Number of Neutrons = Mass Number - Atomic Number

This is the original way the number of particles were calculated. Today there are ways to determine the number of neutrons, electrons and protons directly (similar to the way X-rays were used to determine the number of protons), however I thought this explanation would be a good lesson in the history of science.

I would strongly encourage you to investigate these great discoveries of science and take a closer look into how science and scientific ideas change and evolve as new information is discovered.

Note: the Mass number is simply the atomic mass rounder to the nearest whole mass unit. The atomic mass can be very accurately determined using a variety of techniques that I will not explain here. I would encourage you to look up this information as well.