Final answer:
Alpha particles have a mass of 4 amu because they consist of two protons and two neutrons, each with masses close to 1 amu.
Step-by-step explanation:
The mass of alpha particles, which is 4 atomic mass units (amu), is due to their subatomic makeup. Alpha particles are composed of two protons and two neutrons. Both protons and neutrons are relatively heavy particles with masses close to 1 amu (protons have a mass of approximately 1.0073 amu and neutrons have a mass of approximately 1.0087 amu). The combined mass of these four particles gives alpha particles their total mass of approximately 4 amu. Alpha particles, with a mass of 4 amu, are composed of two protons and two neutrons. Protons have a mass of 1 amu each, and neutrons also have a mass of 1 amu each. Therefore, the subatomic makeup of alpha particles consists of four subatomic particles: two protons and two neutrons.