Beta particles ($\beta$) are electrons, therefore they have a negative charge and are deflected by electric and magnetic fields in the opposite direction to alpha particles. These electrons that some atoms emit originate in the atomic nucleus, through the transformation of a proton into a neutron. Nuclei with a high proton/neutron ratio emit beta particles to stabilize themselves.

Beta particles are represented by the symbol $^{0}_{-1}\beta$, indicating that they have a zero mass number (it has a mass much less than the proton and neutron) and the equivalent of an atomic number of -1 . The beta emission requires the release of another particle called a neutrino ($\nu$), leaving the nuclear equation as follows. \begin{equation} ^{1}_{0}n\rightarrow ^{1}_{1}p+^{-1}_{0}\beta+\nu \end{equation} Thorium-234 decays by beta emission to Protactinium-234, according to the nuclear equation: \begin{equation} ^{234}_{90}Th\rightarrow ^{234}_{91}Pa + ^{-1}_{0}\beta + \nu \end{equation}