Beta decay, by definition, is radioactive decay that releases an electron.
A neutron is slightly more massive than a proton. A neutron’s beta decay results in a down-quark within it becoming an up-quark and a virtual W boson. The W then decays into an electron + antineutrino. A neutron is composed of quarks, udd and protons contain quarks, uud. The neutron then decays to an electron, an electron and an electron-flavored, anti-neutrino.
This decay preserves all relevant quantum numbers.
- Electric charge: The neutron and anti-neutrino have neutral charges; the proton’s 1 balances the electrons’ -1
- Baryon number: The neutron and proton share the same baryon numbers (1). Leptons have a lower baryon count
- Lepton number: The proton and neutron don’t have one; the electron has one. This balances the lepton number (1) of the anti-neutrino (0-1).
- Number of electrons: Same pattern as above