The picture shows a close-up of one carbon atom. A hydrogen atom has one proton as the nucleus and one electron in the region outside the nucleus. The electron and proton are attracted to each other.

This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons ... positive protons but one electron so it is a positive ion. Helium ion, two neutrons, two protons and ...

The mass of an electron is very small ... Hence atomic number = 18 Number of protons + neutrons = 18 + 22 = 40. Hence mass number = 40The chemical symbol for argon with its mass number and atomic ...

He called it a neutron, and imagined it as a paired proton and electron. There was no evidence for any of these ideas. Chadwick kept the problem in the back of his mind while working on other things.