Ernest Rutherford later devised an experiment with his research partner Hans Geiger and with help of Ernest Marsden, that involved the deflection of alpha particles (helium nuclei) directed at a thin sheet of metal foil. In his plum pudding model, Thomson suggested that an atom consisted of negative electrons randomly scattered within a sphere of positive charge. Thomson postulated that there must be a positive charge as well. Knowing that atoms are electrically neutral, J. The electron had already been discovered by J. The nucleus was discovered in 1911, as a result of Ernest Rutherford's efforts to test Thomson's " plum pudding model" of the atom. The branch of physics concerned with the study and understanding of the atomic nucleus, including its composition and the forces that bind it together, is called nuclear physics. These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 26,634 (uranium atomic radius is about 156 pm ( 156 ×10 −12 m)) to about 60,250 ( hydrogen atomic radius is about 52.92 pm). The diameter of the nucleus is in the range of 1.70 fm ( 1.70 ×10 −15 m ) for hydrogen (the diameter of a single proton) to about 11.7 fm for uranium. Protons and neutrons are bound together to form a nucleus by the nuclear force. Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud. An atom is composed of a positively charged nucleus, with a cloud of negatively charged electrons surrounding it, bound together by electrostatic force. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.
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