Balanced nuclear reaction: 234/91 Pa → 4/2 He + 230/89 Ac.
1) number of protons and neutrons on both side of nuclear reaction must be the same. There are 91 protons (atomic number) and 143 neutrons (mass number - atomic number, 234 - 91 = 143 or (4-2) + (230 - 89) = 143)) on both side of reaction.
2) alpha
decay is radioactive decay in which
an atomic nucleus emits
an alpha particle (helium
nucleus) and transforms into an atom with an atomic
number that is reduced by two and mass number that is reduced by four, so atomic mass of new element is 89 (91 - 2) and mass number is 230 (234 - 4 = 230).
3) look at atomic number, element with atomic number 89 is actinium.
