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.
Mister Atom Mister Atom can fly and fire nuclear blasts from his hands (In the Golden Age, at least, lead is impervious to these blasts). Mister Atom appeared in an issue of the "Justice League Unlimited" comic book, where Doctor Sivana was trying to rebuild him with the help of a local gang. Atom and Sivana were eventually defeated by the Justice League, particularly Vibe. The character also appeared several times in the all-ages comic series "Billy Batson and the Magic of Shazam!" Mister Atom Mister Atom is a fictional comic book supervillain, a radioactive robot who is regularly seen as an