The chlorine has one pair of isotopes with same atomic number 17 but different mass number 35 and we take the atomic mass of chlorine as the average mass of 35 and calculating we will get the average as ,in general we take mass of chlorine as 35.5u..
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Chlorine is a isotope with number 17 . But it has a different mass no. 35 and 37 so we take the average of these numbers . By calculating we get 35.5 so we take this as its Atomic mass number..
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