The average relative mass, also known as the average atomic mass, is found by considering the mass and abundance of each isotope of an element. Here's a breakdown of the process:
Calculating Average Relative Mass
The calculation involves three main steps. These steps are crucial for accurately determining the average atomic mass of an element:
Step 1: Identify Isotope Information
First, you must identify all the isotopes of the element. For each isotope, you need to know:
- Percentage Abundance: The proportion of each isotope that exists in a naturally occurring sample of the element. This is usually given as a percentage.
- Isotopic Mass: The mass of a single atom of that isotope.
Step 2: Multiply Mass by Abundance
Next, for each isotope, you multiply its mass by its percentage abundance. If the abundance is given as a percentage, convert it to a decimal by dividing by 100.
Step 3: Sum the Results
Finally, you add up all the results you got from step two. The result is the average atomic mass of the element.
Example Calculation
Let's consider a hypothetical element with two isotopes, 'A' and 'B':
| Isotope | Mass (amu) | Percentage Abundance |
|---|---|---|
| A | 10.0 | 20% |
| B | 12.0 | 80% |
Using the steps above, we calculate the average relative mass as follows:
-
Step 2 Calculation:
- Isotope A: (10.0 amu) * (20/100) = 2.0 amu
- Isotope B: (12.0 amu) * (80/100) = 9.6 amu
-
Step 3 Calculation:
- Average Relative Mass = 2.0 amu + 9.6 amu = 11.6 amu
Therefore, the average relative mass of this hypothetical element is 11.6 amu.
Key Points
- The average atomic mass is a weighted average, meaning that isotopes with higher abundance have a greater impact on the final result.
- The average relative mass shown on the periodic table is a standardized value reflecting naturally occurring isotopic abundances.
- This calculation is crucial in stoichiometry, understanding chemical reactions and for other chemical calculations.
