What is the mass number of an alpha particle?

The mass number of an alpha particle is 4.

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The mass number of a given isotope of an element is how many nucleons an atom of that isotope has. For a nuclear fragment, such as an alpha particle, it's how many nucleons are in that fragment.

An alpha particle contains 2 protons and 2 neutrons, making for a total of 4 nucleons, and a mass number of 4.