Question 2.6: The natural abundances of the three stable isotopes of magne...
The natural abundances of the three stable isotopes of magnesium are
78.99% magnesium-24 (23.98504 amu), 10.00% magnesium-25 (24.9858
amu), and 11.01% magnesium-26 (25.9829 amu). Calculate the atomic
weight of magnesium and compare your value with that given in the
Periodic Table.
Strategy
To calculate the weighted average of the masses of the isotopes, multiply
each atomic mass by its abundance and then add.
\left(\frac{78.99}{100}\times \overset{\overset{magnisum- 24}{\downarrow } }{23.99}\ amu \right) + \left(\frac{10.00}{100}\times \overset{\overset{magnisum- 25}{\downarrow } }{24.99}\ amu \right) + \left(\frac{11.01}{100}\times \overset{\overset{magnisum- 26}{\downarrow } }{25.98}\ amu \right)
18.95 + 2.499 + 2.860 = 24.31 amu
The atomic weight of magnesium given in the Periodic Table to four decimal places is 24.3050.