A nucleus that decays spontaneously by emitting an electron or a positron is said to undergo beta decay. Like alpha decay, this is a spontaneous process, with a definite disintegration energy and half-life.
In β– decay, an electron is emitted by the nucleus.
In β+ decay, a positron is emitted by the nucleus.
In beta-minus decay, a neutron transforms into a proton within the nucleus according to
whereas in beta-plus decay, a proton transforms into neutron (inside the nucleus) via
The above processes show that mass number A of a nuclide undergoing beta decay does not change.