The mass number of an atom is the sum of its protons and neutrons. the most common isotope of argon is ar-40 (40 ar), with a natural abundance of 99.6 %. the mass number for ar-40 is 40 . It is having 18 protons and 22 neutrons. So its mass number is 40 that is equal to sum of total number of protons and neutrons.
Mass number=total number of protons and neutrons
=18+22 =40
