Issued as a cumulative or non-cumulative L/C, the former allows for unused credit amounts to rollover into subsequent periods while the latter maintains a fixed amount of credit available each period.
average atomic mass of magnesium is 24.3 g/mole so 24.3 X 1.56 X1021 / 6.02 X1023 = .063 grams.
Atomic mass is the total mass of protons and neutrons in an individual atom or isotope. Atomic weight is the average mass of all of the naturally-occurring isotopes of an element. Both atomic mass and atomic weight are represented using a unit called the atomic mass unit (amu), which was defined using carbon's most common isotope, carbon-12.