how, when a radio active substance emits an electron, can this come from the nucleus which contains only protons and

Question: how, when a radio active substance emits an electron, can this come from the nucleus which contains only protons and neutrons?
- Keywords:
  nuclear,
  particle,
  radiation
Asked by mojems (imogen) to Susan, Rob, Josh on 17 Jun 2015.
- Susan Cartwright answered on 17 Jun 2015:
  
  Because one of the neutrons decays into a proton, an electron and an antineutrino. The electron and the antineutrino escape, and the proton replaces the neutron, making a different element. For example, carbon-14 (6 protons, 8 neutrons) decays to nitrogen-14 (7 protons, 7 neutrons).

Question: how, when a radio active substance emits an electron, can this come from the nucleus which contains only protons and neutrons?