How to Calculate Relative Atomic Mass
Relative atomic mass (Ar) represents the average mass of an atom of an element, considering the different isotopes and their relative abundances. It's calculated by summing the masses of each isotope multiplied by its relative abundance, then dividing by the total abundance. This weighted average accounts for the fact that most elements exist as a mixture of isotopes.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. This leads to variations in their atomic mass. The relative abundance refers to the percentage of each isotope found in nature.
Calculating Relative Atomic Mass: A Step-by-Step Guide
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Identify the isotopes: Determine which isotopes of the element exist.
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Find their masses: Obtain the mass of each isotope (usually given in atomic mass units, amu).
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Determine their abundances: Find the percentage abundance of each isotope in nature. These percentages should add up to 100%. Convert percentages to decimals (e.g., 75% becomes 0.75).
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Apply the formula: The relative atomic mass (Ar) is calculated using the following formula:
Ar = Σ [(mass of isotope * abundance of isotope)]
Where:
- Σ represents the sum of all isotopes.
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Calculate: Perform the calculation. The result will be the relative atomic mass of the element.
Example Calculation
Let's calculate the relative atomic mass of chlorine, which has two main isotopes:
- Chlorine-35 (³⁵Cl): mass = 35 amu, abundance = 75% (0.75)
- Chlorine-37 (³⁷Cl): mass = 37 amu, abundance = 25% (0.25)
Ar = (35 amu 0.75) + (37 amu 0.25) = 35.5 amu
Therefore, the relative atomic mass of chlorine is 35.5 amu.
Resources and Further Reading:
Several online resources provide further explanation and tools for calculating relative atomic mass:
