An atom consists of a central nucleus surrounded by an electron cloud. The nucleus is a tightly packed ball made up of protons and neutrons. The mass of the proton and the neutron are nearly the same. The mass of the electron is about 1/2000 of that. The proton has a charge of +1. The neutron has a charge of 0. The electron has a charge of -1. The number of protons in the nucleus is called the atomic number. It tells us what the element is. Hydrogen has one proton, helium has two, lithium three, etc. The number of neutrons gives you the isotope of the given element.