What are Isotopes
Isotopes are the same element with different mass numbers. In other words, they have the same number of protons, but different numbers of neutrons. Some isotopes are more common than others.
For example, carbon-12 is a carbon isotope with 6 protons and 6 neutrons. Carbon-14 is a carbon isotope with 6 protons and 8 neutrons. They have the same number of protons, but different numbers of neutrons.
Mass Spectrometry
This graph above was created by a machine called a mass spectrometer, which uses magnetic fields to separate isotopes of an element by mass.
The x-axis indicates the mass of each isotope, and the y-axis reveals the abundance of the isotope (as a percent). Information from this graph can be used to calculate average atomic mass, as shown below.
Calculating Average Atomic Mass
Average atomic mass considers both the mass of each isotope, and their natural relative abundance. This is how you calculate average atomic mass.
X₁M₁ + X₂M₂… = Mavg
Where X represents the relative abundance of an isotope, and M represents the mass of that isotope.
Example Problem
Calculate Average Atomic Mass of Magnesium
| Isotope | Abundance (%) | Atomic Mass (amu) |
|---|---|---|
| Mg-24 | 78.99 | 23.99 |
| Mg-25 | 10.00 | 24.99 |
| Mg-26 | 11.01 | 25.98 |
Answer
0.7899(23.99) + 0.1000(24.99) + 0.1101(25.98) = 24.31 amu, which is the same average atomic mass of Mg listed on the periodic table.
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