The world really isn't made of a collection of fixed little building-block particles. There is no fixed number of electrons, so you don't need one in the nucleus to start with for one to come out.
There are some things that are fixed ("conserved"), though. These include energy, momentum, angular momentum, and electrical charge. So for a nucleus to change, there has to be just the right combination of outgoing particles to conserve all the conserved quantities- the ones listed plus some others. Emission of a combination of an electron, a neutrino, etc, can work to conserve these in beta decay processes.
A fundamental beta decay processes is neutron --> proton + electron + neutrino, e.g. carbon 14 decays into nitrogen 14 plus an electron and a neutrino. The half life of this process is about 5,700 years, useful for ’carbon dating’ of archeaological artifacts.
Why a particular type of nucleus decides to undergo beta decay and how long it typically lives is another question to be asked.
Mike W. and Lee H.
(published on 02/24/07)