<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> 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 . 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 . Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud . Protons and neutrons are bound together to form a nucleus by the nuclear force . </P> <P> The diameter of the nucleus is in the range of 6985175000000000000 ♠ 1.75 fm (6985175000000000000 ♠ 1.75 × 10 m) for hydrogen (the diameter of a single proton) to about 6986150000000000000 ♠ 15 fm for the heaviest atoms, such as uranium . These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 23,000 (uranium) to about 145,000 (hydrogen). </P>

Why is the nucleus smaller than the atom
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