An alpha particle contains 2 protons.

Explanation

• An alpha particle is essentially the nucleus of a helium-4 atom, which consists of 2 protons and 2 neutrons bound together.
• It has a charge of +2 due to the two protons and no electrons.
• The mass of an alpha particle is approximately 4 atomic mass units, from the sum of the 2 protons and 2 neutrons.

Thus, the number of protons in an alpha particle is 2.