Final answer:
Magnesium atoms have different mass numbers due to the presence of different isotopes, which have varying numbers of neutrons. The average mass of a magnesium atom is calculated using the masses and percent abundances of its isotopes, resulting in an average mass of 24.305 amu.
Step-by-step explanation:
The mass number of magnesium atoms differs because they represent different isotopes of magnesium. The mass number is the sum of the number of protons and neutrons in an atom's nucleus. Since all magnesium atoms have 12 protons, the difference in mass number is due to the differing numbers of neutrons. To calculate the average mass of a magnesium atom, you multiply the mass of each isotope by its percent abundance and then add those values together.
For example:
- (78.70% × 23.98 amu) for 24Mg
- (10.13% × 24.99 amu) for 25Mg
- (11.17% × 25.98 amu) for 26Mg
The combined total gives the average mass of a magnesium atom in the sample:
Average mass = (0.7870 × 23.98) + (0.1013 × 24.99) + (0.1117 × 25.98) amu = 24.305 amu (rounded to three decimal places).