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Binding energies are usually expressed in units of electron volts (e V) or million electron volts (Me V) per atom.Binding energies gradually increase with atomic number, although they tend to level off near the end of the periodic table.The excess energy associated with this excited state is released when the nucleus emits a photon in the -ray portion of the electromagnetic spectrum.Most of the time, the -ray is emitted within 10Nuclides with atomic numbers of 90 or more undergo a form of radioactive decay known as spontaneous fission in which the parent nucleus splits into a pair of smaller nuclei.The reaction is usually accompanied by the ejection of one or more neutrons.In 1934 Enrico Fermi proposed a theory that explained the three forms of beta decay.Alpha decay is usually restricted to the heavier elements in the periodic table.
Electron emission therefore leads to an increase in the atomic number of the nucleus.
Alpha decay of the The sum of the mass numbers of the products (234 4) is equal to the mass number of the parent nuclide (238), and the sum of the charges on the products (90 2) is equal to the charge on the parent nuclide.
Nuclei can also decay by capturing one of the electrons that surround the nucleus.
A graph of the number of neutrons versus the number of protons for all stable naturally occurring nuclei.
Nuclei that lie to the right of this band of stability are neutron poor; nuclei to the left of the band are neutron-rich.