The average atomic mass is calculated by the expression:
Average atomic mass = Σzₓ(Aₓ)
where zₓ is the relative abundance of an isotope and Aₓ is the atomic mass of the isotope.
We calculate the mass of the isotope as follows:
10.81 = .1980 (10.012938) + .8020 (x)
x = 11.00 -----------> mass of the isotope
