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Why is atomic mass of chlorine taken as 35.5 u and not a whole number like 35 u or 36 u? Explain.

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Chlorine in natural state found in the mixture of two isotopes with atomic masses 35 u and 37 u in the ratio 3 : 1. Hence, the atomic mass of chlorine is taken as the average of atomic masses of these two isotopes. 

Therefore average atomic mass of Cl = 35 × 3/ 4 + 37 × 1/ 4 = 35.5u

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