The discovery of the nuclear atom by Ernest Rutherford in 1911 was a critical step forward in improving our understanding of the fundamental nature of matter. However, it was immediately clear to scientists that Rutherford's atomic model was incomplete. The simplest and most obvious data showed without question that atoms must consist of more than nuclei, which contain protons, and electrons, located outside the nucleus.

Those data come from measurements of atomic number and atomic mass. An element's atomic number is equal to the total positive charge on--the number of protons in--the nuclei of atoms of which the element is made. The element's atomic mass is equal to the total mass of all particles that make up the atom. Since the mass of a single proton on the atomic mass scale is 1, and that of the electron is 0.0055, the mass of the atom must be very nearly equal to...