An electron capture occurs within the nucleus of an atom that has many protons and few neutrons. During this process, one of the protons in the atom's nucleus pulls in an orbiting electron and neutralizes both the electron and itself. This causes the atom to decay and become a different element with the same atomic mass. Electron capture does not occur in all elements and does not occur with protons or electrons that are not part of relatively massive atoms.
Atoms can undergo radioactive decay in a number of different ways, including through an electron capture. This process generally occurs in atoms that do not have enough energy to participate in other types of radioactivity, including positron emission, which occurs through a similar process. The amount of energy needed to use one of these decay processes is determined by comparing the amount of energy in an atom with the amount in a daughter atom that it could decay into. Atoms that capture electrons require less energy than other radioactive atoms.
When an atom decays by the process of electron capture, it pulls one of its own electrons into its nucleus. These electrons are usually taken from either the K or L electron shells, which are the two shells closest to the atom's nucleus. Free protons and electrons cannot interact with each other through the process of electron capture.
Once an electron is pulled into the atom's nucleus it is absorbed by one of the protons. It is this proton that is said to capture the electron. The reaction that occurs between the proton and the electron causes these two particles to neutralize, resulting in a neutron and a neutrino, which is a small particle similar to an electron though lacking in charge. The neutron remains in the atom's nucleus while the neutrino is expelled from the atom.
After an electron capture occurs, the atom is a different element than it started out as. This daughter atom will be the element that directly preceded the parent atom on the periodic table because it will have one less proton in the nucleus, and atoms are defined by the number of protons they have. Changing one element into another causes the atom to become excited. This extra energy is released through the release of an extra electron, an x-ray, and sometimes a gamma ray. The release of an x-ray and gamma ray are what make the process of electron capture radioactive.