Write a balanced nuclear reaction for the following:
234/91 pa → 4/2 he+

when balancing the nuclear reaction, explain the following:
how does the law of conservation of matter dictate what the daughter nuclide is?
how do we calculate the atomic mass and atomic number for the daughter nuclide?
where do we look up what the new daughter nuclide element is?

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.