Here's why:

* Alpha particles consist of 2 protons and 2 neutrons.

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

* When an alpha particle is emitted, the atom loses 2 protons, hence reducing the atomic number by 2.

Example:

* Uranium-238 (atomic number 92) emits an alpha particle to become Thorium-234 (atomic number 90).