The change in mass number is 232 - 220 = 12. This requires 12/4 = 3 alpha particles. Ejecting 3 alpha particles decreases the atomic number by 3×2=6, from 92 to 86. To reach the final atomic number of 89, the atomic number must increase by 3. This is achieved by the emission of 3 beta particles. [cite: 1, 1608]