The lesson involves answering the question, "why is the radius of a uranium atom (U-238) only about 3x greater than the radius of a hydrogen atom (H-1). The answer was arrived at through a discussion of electric field attraction between nuclear protons and orbiting electrons, and how the addition of more protons and electrons affects the innermost orbits of electrons, as more electrons are added, the inner orbits shrink in size due to stronger electrical attraction.