An alpha particle. It is also a helium nucleus and has a charge
of +2e. The ejection of an alpha particle occurs whenever the
electromagnetic force overcomes the strong force in a nucleus and
needs to rid itself of excess charge. The reason that a single
proton is not simply ejected has to do with the stability of the
alpha particle itself which is generated by the strong force. If
there were more protons in an alpha particle, it would not hold
together properly. It cannot have more neutrons because it simply
wouldn't be ejected because of strong force.
