Atom bombardment ejects what appeared to be hydrogen nuclei. By 1920 he had accepted that the hydrogen nucleus is a distinct particle within the atom and named it proton. Neutrons have no electrical charge and have a free mass of 1,839 times the mass of the electron, or , the heaviest of the three constituent particles, but it can be reduced by the nuclear binding energy. Neutrons and protons (collectively known as nucleons) have comparable dimensions—on the order of —although the 'surface' of these particles is not sharply defined. The neutron was discovered in 1932 by the English physicist James Chadwick.
A Is for Atom tighter editing, new music and a focus on nuclear power as opposed to nuclear bombs. A narrator is relating what is an atom and how atomic energy can be harnessed by man to produce "limitless" energy. Dr. Atom (a caricature with an atom for a head) then explains the similarities between the solar system and atomic structure. He then goes on to relate how the atom is made up of protons, neutrons, and electrons. After this, the narrator explains how there are more than 90 elements with many possible isotopes for each. The history of atomic energy is then overviewed