The atomic mass of an element is the weighted average mass of all its isotopes, while the relative abundance of an isotope is the percentage of that specific isotope found in a naturally occurring sample of the element.
Here's a more detailed breakdown:
Atomic Mass (Relative Atomic Mass)
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Definition: Atomic mass, more accurately termed "relative atomic mass," is the average mass of an element calculated by considering the mass and abundance of all its naturally occurring isotopes. It's a weighted average, meaning that more abundant isotopes contribute more to the overall atomic mass.
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Units: Expressed in atomic mass units (amu or u). Often, but not always, close to a whole number.
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Calculation: The atomic mass is calculated using the following formula:
Atomic Mass = (Isotope 1 Mass × Isotope 1 Abundance) + (Isotope 2 Mass × Isotope 2 Abundance) + ...
Where:
- Isotope Mass is the mass of a specific isotope.
- Isotope Abundance is the relative abundance of that isotope (expressed as a decimal).
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Example: Consider chlorine (Cl), which has two main isotopes: Chlorine-35 (35Cl) and Chlorine-37 (37Cl). Chlorine-35 has a mass of approximately 34.97 amu and a relative abundance of 75.76% (0.7576). Chlorine-37 has a mass of approximately 36.97 amu and a relative abundance of 24.24% (0.2424).
Therefore, the atomic mass of chlorine is:
(34.97 amu × 0.7576) + (36.97 amu × 0.2424) = 35.45 amu
Relative Abundance of Isotopes
- Definition: Relative abundance is the percentage or proportion of each specific isotope present in a naturally occurring sample of an element. It indicates how commonly a particular isotope is found.
- Units: Expressed as a percentage (%) or a decimal fraction.
- Key Points:
- The sum of the relative abundances of all isotopes of an element must equal 100% or 1.
- Relative abundances are determined experimentally, usually through mass spectrometry.
- They are relatively constant across different samples of the element, although slight variations can occur depending on the source.
- Example: As mentioned above, Chlorine-35 has a relative abundance of 75.76% and Chlorine-37 has a relative abundance of 24.24%. This means that if you were to analyze a large sample of chlorine, approximately 75.76% of the chlorine atoms would be Chlorine-35, and 24.24% would be Chlorine-37.
Summary Table
| Feature | Atomic Mass (Relative Atomic Mass) | Relative Abundance of Isotopes |
|---|---|---|
| Definition | Weighted average mass of all isotopes of an element. | Percentage or proportion of a specific isotope in a natural sample. |
| Focus | Element as a whole (considering all isotopes). | Individual isotopes. |
| Units | amu (u) | Percentage (%) or decimal fraction. |
| Determined By | Calculated from isotope masses and their relative abundances. | Experimentally determined (e.g., mass spectrometry). |
In essence, atomic mass reflects the average mass of an element's atoms, considering the contribution of each isotope based on how common it is (its relative abundance). Relative abundance, on the other hand, describes the distribution of different isotopes for a given element.
