The mass number of barium is 138. Barium is a chemical element with the symbol Ba and is located in group 2 of the periodic table. The mass number represents the total number of protons and neutrons in the nucleus of an atom. For barium, the most stable and abundant isotope is barium-138. This means that barium has 56 protons (which is its atomic number) and 82 neutrons, adding up to a mass number of 138.
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