Here's why:

* Alpha particles are essentially helium nuclei. They consist of two protons and two neutrons.

* Protons and neutrons have nearly the same mass. Each has a mass of approximately 1 amu (atomic mass unit).

* Therefore, the mass of an alpha particle is roughly 2 protons + 2 neutrons = 4 amu.

The actual mass is slightly more than 4 amu due to the binding energy of the nucleus, which is a small amount of mass converted to energy.