Question

What is the change in atomic mass number when an atom emits an alpha particle?

Answer 1

An alpha particle is nothing else but a Helium-4 nucleus.

Explanation:

So the atom loses two protons and two neutrons.
The atomic number Z (=its place in the periodic table) will go down by 2 (the protons), and the mass number A will go down by 4 (protons+neutrons).