Elements with lower and higher mass numbers per nucleon are less stable. The total mass of a nucleus is less than the total mass of the nucleons that make up the nucleus. This difference is known ...

However as the number of protons increases, more neutrons are needed to keep the nucleus stable. For example lead, lead-206 has 82 protons and has 124 neutrons.

Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional ...

The isotope lead-208 was predicted to be extremely stable and perfectly spherical because of the “magic” numbers of electrons and protons orbiting its nucleus. When researchers blasted lead ...