The average atomic mass of chlorine is 35.45 u (atomic mass units). This is a weighted average reflecting the relative abundance of chlorine's two main isotopes, chlorine-35 and chlorine-37.
Understanding Isotopes and Weighted Averages
Chlorine, like many elements, exists as a mixture of isotopes. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This results in different atomic masses for each isotope. To find the average atomic mass, we need to consider the mass of each isotope and its relative abundance (percentage).
- Chlorine-35 (³⁵Cl): This isotope has a mass of approximately 34.9689 u and makes up about 75.77% of naturally occurring chlorine.
- Chlorine-37 (³⁷Cl): This isotope has a mass of approximately 36.9659 u and accounts for approximately 24.23% of naturally occurring chlorine.
Calculation
The average atomic mass is calculated using a weighted average:
(abundance of isotope 1 × mass of isotope 1) + (abundance of isotope 2 × mass of isotope 2) + ...
For chlorine:
(0.7577 × 34.9689 u) + (0.2423 × 36.9659 u) = 35.45 u
Therefore, the average atomic mass of chlorine is approximately 35.45 u. Slight variations may occur depending on the precise values used for isotopic masses and abundances.
Examples from References
Several sources confirm this calculation:
- Toppr: Provides a calculation resulting in 35.45 g/mol, which is equivalent to 35.45 u.
- Chemistry LibreTexts: Shows the calculation steps, using similar isotopic abundances and masses.
- Byjus: States that the mass number of chlorine is 35.5, which represents the average atomic mass.
- Socratic, Brainly, Vedantu, Quora, Reddit: These various online forums and question-answer sites also confirm the average atomic mass of chlorine to be approximately 35.45 u.
