liquid-drop model, in nuclear physics, a description of atomic nuclei in which the nucleons (neutrons and protons) behave like the molecules in a drop of liquid. If given sufficient extra energy (as by the absorption of a neutron), the spherical nucleus may be distorted into a dumbbell shape and then split at the neck into two nearly equal fragments, releasing energy. Although inadequate to explain all nuclear phenomena, the theory underlying the model provides excellent estimates of average properties of nuclei. Russian-born American physicist George Gamow formulated the model in 1929, and Austrian physicists Lise Meitner and Otto Frisch used it in 1938 to explain nuclear fission.