Atomic mass is equal to the average number allowing for the relative abundances of different isotopes. It is calculated by:
Atomic mass = ΣzₓAₓ
where zₓ is the relative abundance of the isotope and Aₓ is the atomic mass of the isotope.
Atomic mass = (0.0946x85.91) + (0.07x86.91) + (0.8354x87.91) = 87.6508 amu
