Some nuclei are unstable because they have too many neutrons. One way they can become more stable is by beta-minus (β-) decay. In this process a neutron in the nucleus changes into a proton and an electron.
The new proton stays in the nucleus, but the electron is emitted from the nucleus at high speed. This fast electron is called a beta particle. It does not come from the electron shells around the atom.
Because a neutron becomes a proton, the atomic (proton) number goes up by 1. The mass (nucleon) number stays the same, because the total number of protons and neutrons has not changed. For example, carbon-14 (atomic number 6) decays to nitrogen-14 (atomic number 7): 146C → 147N + 0-1e.
