How can a proton form a neutron?

[Matriculator]

This is related to a homework problem but I want to understand it as well. How can a proton break up into a positron and neutron when a neutron clearly has a greater mass than a proton?

 

[Astronuc]

Positron emission occurs in proton-rich nuclei, although in many, electron capture is also possible.

An isolated proton would not decay by positron emission, but an isolated neutron will spontaneously decay.

 

[jtbell]

Consider ²²Na which undergoes β⁺ decay to ²²Ne.

²²Na has 11 protons and 11 neutrons, whereas ²²Ne has 10 protons and 12 neutrons. Nevertheless, the mass of a ²²Na nucleus is greater than the sum of the masses of a ²²Ne nucleus and an electron, because of the different binding energies of ²²Na and ²²Ne.

 

[metamathphys]

An isolated proton can not decay since is the lightest particle with baryon number equals to one.
A proton in the presence of other stuff like other protons or neutrons so for instance a proton that is making up the nuclei of some atom may ''borrow" energy from this environment and then it can decay (always conserving baryon number and charge and so on) so one (very) possible outcome will be positron and neutron