Here's why:

* Alpha particle: An alpha particle is essentially a helium nucleus, consisting of 2 protons and 2 neutrons.

* Atomic number: The atomic number represents the number of protons in an atom's nucleus.

* Emission: When an atom undergoes alpha emission, it loses an alpha particle. This means it loses 2 protons.

Therefore, the atomic number of the atom decreases by 2 after alpha emission.