How does positron decay work?
In nuclear reactions, some fundamental quantities are conserved. One of these is charge.
Before the reaction, the net charge is the charge of the proton which is
Another quantity that is conserved is baryon number. Baryons (protons, neutrons) have a baryon number of
Before the reaction, the baryon number of the proton is
The final quantity conserved is lepton number. The proton isn't a lepton, so has no lepton number. Neither does the neutron. The positron is an antimatter electron so has a
For these reasons a proton can decay into a neutron and an electron neutrino.
The overall mass of the nucleus doesn't change, though the atomic number decreases by one, because a proton is lost to become a chargeless neutron.