Beta emission

A beta particle is essentially an electron that’s emitted from the nucleus. Iodine-131 (I-131), which is used in the detection and treatment of thyroid cancer, is a beta particle emitter:

Here, the iodine-131 gives off a beta particle (an electron), leaving an isotope with a mass number of 131 (131 – 0) and an atomic number of 54 (53 – (–1)). An atomic number of 54 identifies the element as xenon (Xe). Notice that the mass number doesn’t change in going from I-131 to Xe-131, but the atomic number increases by one.

In beta emission, a neutron in the nucleus is converted (decayed) into a proton and an electron, and the electron is emitted from the nucleus as a beta particle. Isotopes with a high neutron/proton ratio often undergo beta emission, because this decay mode allows the number of neutrons to be decreased by one and the number of protons to be increased by one, thus lowering the neutron/proton ratio.