The average atomic mass of a chemical element depends on the atomic masses of its naturally occurring isotopes. The identity of a chemical element depends on the number of protons an atom has in its nucleus which is known as atomic number. Any atom that has 19 protons in its nucleus will be an atom of argon.
mass number=A=Z+no. of neutrons no. of neutrons=A−Z=40−18=22
The average atomic mass of the element argon will be
avg. atomic mass = ∑ i (isotope i ×abundance i )
Average atomic mass=35.96755 u×0.003336+37.96273 u×0.000629+39.96238 u×0.996035
Average atomic mass =39.948 u