Some unstable nuclei have too many protons for the number of neutrons they contain. One way they become more stable is by β+ decay (beta plus decay). In β+ decay, a proton in the nucleus changes into a neutron and a positron. The positron is then emitted from the nucleus at high speed.
A positron is the antiparticle of the electron. It has the same mass as an electron, but its relative charge is +1 instead of -1. Charge is conserved: the proton had a charge of +1, and the neutron (0) and positron (+1) add up to +1.
The nucleus ends up with one proton fewer and one neutron more. So the atomic (proton) number goes down by 1, but the mass (nucleon) number stays the same, because the total number of protons and neutrons has not changed. For example, carbon-11 (6 protons) changes into boron-11 (5 protons) by β+ decay.
Do not mix this up with β- decay, where a neutron becomes a proton plus an electron.
