Magnesium has three naturally occurring isotopes: Mg-24 with mass 24.3050 amu and a natural abundance of 78.99 %, Mg-25 with mass 24.9858 amu and a natural abundance of 10.00 %, and Mg-26 with mass 25.9826 amu and a natural abundance of 11.01 %.

[24.3050*(0.7899)] + [24.9858*(0.1000)] + [25.9826*(0.1101)] = ? = atomic mass Mg

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24.55778376

55.3

24.31 amu

To calculate the atomic mass of magnesium, we need to consider the contribution of each isotope. Here's how you can calculate it:

1. Multiply the mass of each isotope by its natural abundance (expressed as a decimal).

For Mg-24:
Mass = 24.3050 amu
Natural abundance = 78.99% = 0.7899
Mass contribution = 24.3050 amu * 0.7899 = 19.2237 amu

For Mg-25:
Mass = 24.9858 amu
Natural abundance = 10.00% = 0.1000
Mass contribution = 24.9858 amu * 0.1000 = 2.4986 amu

For Mg-26:
Mass = 25.9826 amu
Natural abundance = 11.01% = 0.1101
Mass contribution = 25.9826 amu * 0.1101 = 2.8659 amu

2. Add up the mass contributions of all the isotopes.

Total atomic mass = Mass contribution of Mg-24 + Mass contribution of Mg-25 + Mass contribution of Mg-26
Total atomic mass = 19.2237 amu + 2.4986 amu + 2.8659 amu
Total atomic mass = 24.5882 amu

Therefore, the atomic mass of magnesium is approximately 24.5882 atomic mass units (amu).