Question

Summarize: in general, how can you determine the mass number of the daughter product after alpha decay has taken place? How can you determine the atomic number?

Answer 1

An alpha particle is a helium nucleus ... a bundle of
two protons and two neutrons.

So ...

After an atom expels one alpha particle, if nothing else happens
to the nucleus, its new atomic mass is 4 less than it was originally,
and its atomic number is 2 less than it was originally ... it's now an
atom of a different element.