In beta-minus (${\beta }^{-}$) decay, a neutron inside the nucleus is transformed into a proton, an electron (the beta particle), and an antineutrino: $n\to p+e^{-}+{\bar{\nu }}_e$ The resulting nucleus has one more proton than the original, so the atomic number $Z$ increases by 1 ($Z\to Z+1$), while the mass number $A$ remains unchanged.