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.
Embedded atom model a simulation, the potential energy of an atom, formula_1, is given by where formula_3 is the distance between atoms formula_1 and formula_5, formula_6 is a pair-wise potential function, formula_7 is the contribution to the electron charge density from atom formula_5 of type formula_9 at the location of atom formula_1, and formula_11 is an embedding function that represents the energy required to place atom formula_1 of type formula_13 into the electron cloud. Since the electron cloud density is a summation over many atoms, usually limited by a cutoff radius, the EAM potential is a multibody potential. For a single element system