In beta minus decay a neutron in an unstable nucleus changes into a proton and an electron. The electron is emitted at high speed. This is the beta particle, written as β-. The proton stays in the nucleus.
This changes the nucleus. It has one more proton, so the atomic number (proton number) goes up by 1. The total number of protons and neutrons stays the same, because one neutron has been replaced by one proton, so the mass number (nucleon number) does not change. The nucleus has become an atom of a different element.
For example, carbon-14 has 6 protons and 8 neutrons. After beta minus decay it has 7 protons, so it is nitrogen-14: 146C → 147N + 0-1e.
