Here's why:

* Alpha decay is a type of radioactive decay where an unstable atomic nucleus emits an alpha particle, which is essentially a helium nucleus (two protons and two neutrons).

* This emission results in the original nucleus losing two protons, thereby decreasing its atomic number by two.

* The mass number of the nucleus also decreases by four, as it loses two protons and two neutrons.

Example:

Uranium-238 undergoes alpha decay to form Thorium-234:

```

²³⁸U → ²³⁴Th + ⁴He

```

In this reaction, uranium (atomic number 92) loses two protons and becomes thorium (atomic number 90), while emitting an alpha particle (⁴He).