In some cases the probabilities sum to a number greater than 100 percent. The abbreviations used in the table are given below. Single alpha decay results in a decrease of the mass number by four and the atomic number by two. Alpha particles can be stopped by a sheet of paper, and are not dangerous to living things unless the alpha-emitting substance is inhaled, ingested or decays while in contact with the eye lens. Positron decay is always accompanied by electron capture decay.
Beta particles penetrate more than alpha particles but less than gamma rays or x-rays.
If an electron is emitted from the nucleus, one neutron is converted to a proton, increasing the atomic number by one, while the mass number remains essentially unchanged. If a positron is emitted from the nucleus, a proton will change to a neutron, decreasing the atomic number by one and leaving the mass number unchanged. An unseen neutrino accompanies each beta decay. Electron capture occurs when a lower orbital electron is captured by the positively charged nucleus. If the decay energy is greater than 1022 keV, positron emission can also occur in competition with electron capture.