Which one of the following is the average atomic mass of chlorine atom ?

The average atomic mass of chlorine is approximately 35.5 u. This value is a weighted average derived from its two naturally occurring stable isotopes: Chlorine-35 and Chlorine-37 [1]. In nature, these isotopes exist in a relative abundance ratio of approximately 3:1, where Chlorine-35 accounts for about 75% and Chlorine-37 accounts for about 25%. Specifically, the calculation involves multiplying the mass of each isotope by its fractional abundance and summing the results: (35 × 0.75) + (37 × 0.25) = 35.5 u. While the precise standard atomic weight is often cited as 35.45 u in advanced scientific contexts, 35.5 u is the standard value used in general chemistry and textbooks to represent the mass of chlorine atoms in bulk.

Sources

1. [1] https://en.wikipedia.org/wiki/Isotopes_of_chlorine