why atomic mass of some element  are in fraction

Dear user!

Your query is quite obvious as you have studied that atomic mass of any element depends upon the number of protons and neutrons present in it. Therefore, the atomic mass of a substance should always be a whole number.

However, there are some elements which deviate from this.

This is because; they exist in the form of isotopes in nature. Chlorine is an example of one such element. It is having a fractional atomic mass i.e., 35.5.

This is because; in nature chlorine exists in two isotopic forms. These are. These two isotopes have a natural abundance ratio of 3:1. This means that 75.4% of chlorine found in nature exists as isotope, and the rest 24.6% exists as  isotope.

To calculate the average mass of the element, mass as well as the abundance of all isotopic forms are taken into consideration.

Hence, the mass of chlorine can be calculated as follows:

I hope, now it is clear that the value, 35.46 u, indicates the average mass of chlorine atom due to the presence of both the isotopes in a certain amount.

All the best!!