Isotopes contribute to the average atomic mass of an element by influencing it in proportion to their abundance.
Understanding Atomic Mass and Isotopes
To grasp how isotopes affect the average atomic mass, let's clarify a few concepts:
- Atomic Mass: The mass of an atom, usually measured in atomic mass units (amu).
- Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons, leading to different atomic masses.
The Weighted Average
The atomic mass reported on the periodic table is not just a simple average of all the isotope masses. It is a weighted average. This means the abundance (percentage) of each isotope is considered. As the reference highlights:
The quoted atomic mass on the Periodic Table is the WEIGHTED average of the individual isotopic masses. The higher the isotopic percentage, the MORE that isotope will contribute to the isotopic mass. For this reason, most masses that are quoted on the Table are non-integral.
How to Calculate the Average Atomic Mass
- Identify the isotopes: Determine all the existing isotopes of an element.
- Find isotopic masses: Locate the atomic mass of each isotope.
- Determine the percent abundance: Find the natural abundance of each isotope in percentage form.
- Calculate contribution: For each isotope, multiply its isotopic mass by its fractional abundance (percentage / 100).
- Sum the contributions: Add all the individual contributions to obtain the weighted average or average atomic mass.
Example
Let's consider the element chlorine (Cl):
| Isotope | Mass (amu) | Abundance (%) | Calculation | Contribution (amu) |
|---|---|---|---|---|
| Chlorine-35 | 34.9688 | 75.77 | 34.9688 amu * (75.77/100) | 26.4971 |
| Chlorine-37 | 36.9659 | 24.23 | 36.9659 amu * (24.23/100) | 8.9570 |
| Total | 100 | Weighted Average Sum | 35.4541 |
The average atomic mass of Chlorine is approximately 35.45 amu, which aligns with what we find on the periodic table.
Why It Matters
This calculation method explains why many atomic masses on the periodic table are not whole numbers. It directly reflects the natural presence and abundance of each of an element's isotopes. As the percentage of an isotope increases, so will the impact on the overall average atomic mass.
