Here's why:

* Alpha particles are essentially helium nuclei, consisting of two protons and two neutrons.

* Alpha decay occurs when an unstable atom emits an alpha particle. This process reduces the atomic number of the atom by two and the mass number by four.

Example:

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

```

²³⁸U → ²³⁴Th + ⁴He

```

The emitted alpha particle is a helium nucleus (⁴He).