Is atomic weight a whole number or not a whole number

Atomic mass of an element is not a whole number because It contains isotopes. For example, chlorine has two isotopes 1735Cl and 1737Cl with natural abundances in the approximate ratio of 3:1. Hence, the average atomic mass of chlorine is approximately 35.5 g/mol.

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the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.