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Mass no is the no. of neutrons and no. of protons added. Then how come chlorine has a mass no. 35.5?
Chlorine has an atomic mass of 35.5 because the atomic mass of an element is the number of protons plus the number of neutrons. In chlorine, there are always 17 protons (the number of protons in an atom is what makes it whichever element it is), but it is possible to have different amounts of neutrons in the atom. These are called different isotopes of an element. In chlorine, there can be between 17 and 20 neutrons meaning the mass number can range between 34 and 37. The reason that it is written as 35.5 on the periodic table is because that is the average mass number.


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