Beta particles are electrons that are emitted from the nucleus of an atom during beta decay. When a neutron in the nucleus of an atom changes into a proton, it emits an electron and an antineutrino. This electron is called a beta particle. Beta particles have a negative charge and a relatively small mass, about 1/1836th that of a proton. Beta particles can be easily stopped by a few millimeters of aluminum or a few centimeters of air. Beta particles can be emitted by a variety of radioactive isotopes and can pose a health hazard if ingested or inhaled.