In $$\beta^{-}$$decay, the mass number A remains unchanged but the atomic number Z of the nucleus goes up by $$1.$$ A common example of $$\beta^{-}$$decay is
$$^{32}_{15}P \rightarrow ^{32}{16}S + e^{-} + \overset{-}{v}$$
A neutron of nucleus decays into a proton, an electron and an antineutrino. It is this electron which is emitted as $$\beta^{-}$$ particles.
$$^{1}_{0}n \rightarrow ^{1}_{1}p + ^{0}_{-1}e + \overset{-}{v}$$
In $$\beta$$-decay, particles like antineutrinos are also emitted along with electrons. The available energy is shared by electrons and antineutrinos in all proportions. That is why all electrons emitted during $$\beta^-$$decay not have the same energy.