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.
Magnesium has three stable isotopes, Mg-24, Mg-25, Mg-26. The most common isotope is Mg-24, which is 79% of all Mg found on Earth. Mg25 and Mg26 are used to study the absorption and metabolism of magnesium in the human body.