Relative atomic mass of Chlorine is not a whole number; why?

In today's blog post, we have a very important question from one of our students, as regards atomic mass of Chlorine (Cl). 

Why is the atomic mass of chlorine isn't a whole number?


Chlorine(Cl) is originally derived from the Greek word, 'Chloros' meaning greenish-yellow gas, and was discovered in 1774 by the Swedish chemist Carl Wilhelm Scheele.

Chlorine has an atomic number 17 and a mass number of 35.5, according to the periodic table of elements.

The reason why the atomic mass of chlorine is taken as 35.5 (decimal point) and not 36 or 35(whole number) is because of its isotopes - atoms of the same Cl element with the same number of protons(17) but a different mass number or neutrons number.

The isotopes are 35-Cl and 37-Cl with about 75% and 25% abundance in nature respectively.

Generally, the atomic mass of a species (atom or group of atoms) is the sum of the masses of protons, neutrons, and electrons. But for an element, the atomic mass is the average mass of the masses of the naturally occurring isotopes of that element relative to the mass of carbon-12. That is why it is called relative atomic mass.

Mathematically, the relative atomic mass of chlorine =(75/100)x35 + (25/100)x37 = 35.5 

Fig 1: Chlorine symbol. 

Hence, the relative atomic mass of chlorine is not a whole number.



We hopes that help in understanding why atomic mass of Cl is not a whole number. Thank you


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