Radioisotopes are radioactive isotopes of an element. Atoms that contain an unstable combination of neutrons and protons, or excess energy in their nucleus.
On the basis of the 1909 Geiger-Marsden gold foil experiment, Ernest Rutherford identified the atomic nucleus in 1911, which is the compact, dense region made up of protons and neutrons at the heart of an atom. Dmitri Ivanenko and Werner Heisenberg immediately created models for a nucleus made of protons and neutrons after the neutron was discovered in 1932. A positively charged nucleus and a cloud of negatively charged electrons that are held together by electrostatic force make up an atom. The electron cloud contributes very little to the atom's mass, which is almost entirely in the nucleus. The nuclear force brings protons and neutrons together to create a nucleus.
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