As a result of the work of Thomson, Rutherford and others, we know a great deal about the makeup of the atom. We know that each atom has a small positive center called the nucleus surrounded by electrons. Further, we know that the nucleus is comprised of smaller particles called protons and neutrons.
So, in simplest terms, the protons determine the identity of the atom, the neutrons (along with the protons) determine the mass of the atom and the electrons determine the charge (in comparison to the number of protons) of the atom or ion. Knowing these relationships allows us to do some simple calculations.
For the rest of the unit, we will use the standard shorthand way of representing atoms and particles which is described here.