The average atomic mass of an element is calculated by summing the masses of its isotopes, each multiplied by its natural abundance. This can be represented by the following formula:
Average Atomic Mass = (Mass of Isotope 1 × Abundance of Isotope 1) + (Mass of Isotope 2 × Abundance of Isotope 2) + ...
- Isotopes: Atoms of the same element with the same number of protons but a different number of neutrons.
- Natural Abundance: The percentage of each isotope that naturally occurs in a sample of the element.
Example:
Consider chlorine, which has two main isotopes: chlorine-35 and chlorine-37.
- Chlorine-35: Mass = 34.9689 amu, Abundance = 75.77%
- Chlorine-37: Mass = 36.9659 amu, Abundance = 24.23%
To calculate the average atomic mass of chlorine:
- Convert percentages to decimals: 75.77% = 0.7577 and 24.23% = 0.2423
- Multiply the mass of each isotope by its abundance:
- (34.9689 amu × 0.7577) + (36.9659 amu × 0.2423)
- Add the results: 26.50 amu + 8.95 amu = 35.45 amu
Therefore, the average atomic mass of chlorine is approximately 35.45 amu.
In summary: The average atomic mass formula reflects the weighted average of the masses of an element's isotopes, taking into account their relative abundance. This value is often used in chemical calculations and represents the mass of a typical atom of that element.
