The mass defect of an atom reflects the stability of the nucleus.
It is equal to the energy released when the nucleus is formed from its protons and neutrons.
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.
It is therefore literally the energy that binds together the neutrons and protons in 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.
He argued that a neutron could decay to form a proton by emitting an electron.
A proton, on the other hand, could be transformed into a neutron by two pathways.