Here's how alpha decay affects an atom's mass and atomic number:

* Atomic Number: Decreases by 2. An alpha particle consists of 2 protons and 2 neutrons (essentially a helium nucleus). When emitted, the atom loses these protons, reducing its atomic number.

* Mass Number: Decreases by 4. The alpha particle has a mass number of 4 (2 protons + 2 neutrons). Losing this particle reduces the atom's mass number by 4.

In summary:

* Alpha Decay: Atomic Number - 2, Mass Number - 4

Let me know if you'd like a specific example of alpha decay!