Beta decay

In nuclear physics, beta decay is a type of radioactive decay where a beta particle (an electron or a positron) is made and sent away, or emitted. When it is an electron, it is called beta minus, and changes a neutron into a proton. When it is a positron, it is beta plus and changes a proton into a neutron. The weak force makes Beta decay possible.

In regular matter^†, as the neutron becomes a proton, but the number of nucleons stays the same, the atomic mass will change by 0, but the atomic number will increase by +1. In effect, we can say the β particle has an atomic number of "-1".$${\ce {^{16}_{6}C -> ^{16}_{7}N + ^{0}_{-1}\beta}}$$(† - see Antimatter)