When a radioactive isotope releases a beta particle, the atomic number of the new element is?

β- decay is, when a neutron emits an electron and an antineutrino and is thereby "transformed" into a proton. As the number of protons determines the element, the nucleus now resembles an isotope of another element than before.

However, unstable atomic nuclei with an excess of protons may undergo β+ decay, where a proton is converted into a neutron, a positron, and a neutrino.