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.
Further data for naturally occuring isotopes of magnesium are listed above. This table gives information about some radiosotopes of magnesium, their masses, their half-lives, their modes of decay, their nuclear spins, and their nuclear magnetic moments. Isotope Mass / Da Half-life Mode of decay Nuclear spin Nuclear magnetic moment; 22 Mg: 21.999574: 3.86 s