Answer:
Reduce the mass number of the parent by 4
Explanation:
To determine the mass number of the daughter product after alpha decay has taken place, reduce the mass number of the parent by 4.
An alpha particle reduces the mass number of the parent by 4 and the atomic number by 2.
It has the the same structure as an helium atom.
So, since helium has mass number of 4 and atomic number of 2, it reduces the daughter product by this number.
