The atomic mass of chlorine is calculated using a weighted average of the masses of its isotopes, taking into account their relative abundances. Chlorine has two naturally occurring isotopes: Chlorine-35 (35Cl) and Chlorine-37 (37Cl).
Understanding Isotopes and Atomic Mass
- Isotopes: Atoms of the same element with the same number of protons but a different number of neutrons. This results in different atomic masses for the isotopes of an element.
- Atomic Mass: The weighted average of the masses of all isotopes of an element, taking into account their natural abundance. This is the value typically listed on the periodic table.
Calculating Chlorine's Atomic Mass
The calculation involves the following steps:
-
Identify Isotopes and Abundances: Chlorine has two main isotopes:
- 35Cl: Atomic mass ≈ 34.969 amu, Abundance ≈ 75.77%
- 37Cl: Atomic mass ≈ 36.966 amu, Abundance ≈ 24.23%
-
Perform Weighted Average Calculation: The average atomic mass is calculated by multiplying the atomic mass of each isotope by its relative abundance (expressed as a decimal), and then summing the results. The formula is:
Average Atomic Mass = (Mass of Isotope 1 × Abundance of Isotope 1) + (Mass of Isotope 2 × Abundance of Isotope 2) + ...
For Chlorine:
Average Atomic Mass = (34.969 amu × 0.7577) + (36.966 amu × 0.2423)
≈ 26.496 amu + 8.954 amu
≈ 35.45 amu
Therefore, the atomic mass of chlorine is approximately 35.45 amu (atomic mass units), or 35.45 g/mol (grams per mole). This value is a weighted average reflecting the relative proportions of each isotope in naturally occurring chlorine.
