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.
Captain Atom of alternate Captains Atom, including Earth-13's Brigadier, the Captain Atom as depicted in the graphic novel "Kingdom Come" (Earth-22), and a Captain in a red/silver variant of the Monarch's costume, one similar to the Charlton Comics Atom (Earth-4), and a Hulked out variant named Attum from an unknown Earth. Several more variants are shown in "Countdown: Arena" #4, including a President Atom, a robot called Quantum Mechanix, Kid Quantum of the Legion of Super-Heroes, Captain Adamma, Quantum Boy, an anthropomorphic wolf version, a from Earth-30, a Doctor Manhattan-lookalike, an energy based-Atom who makes calculations during his attacks, and a giant-sized