Too many, or not enough neutrons in a nucleus can lead to an unstable nucleus. In order to become more stable, radiation is produced in one of several ways: Too many neutrons in a nucleus and a neutron will change into a proton and eject a high energy electron (beta particle). Too few neutrons, and a proton will decay into a neutron with the release of a positron, the anti particle of an electron.
Alpha emission occurs in heavy nuclei such as uranium, where the nucleus is unstable due to its mass. An alpha particle consists of 2 neutrons and 2 protons, also defined as the nucleus of a helium atom.