Why we are write mass of chlorine atom as 35.5 not like other atom?
For example : Na=23u ,C=12 ,O=16u .

It is due to the relative abundances of its isotopes

Explanation:

There are two principal Chlorine isotopes. About 75% of Chlorine atoms have a relative atomic mass of 35 and 25% have a relative atomic mass of 37.

Hence in a sample of Chlorine, the average relative atomic mass would be:
(75%×35)+(25%×37)=35.5

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