Taking into account the definition of atomic mass, isotopes and atomic mass of an element, the average atomic mass is 35.5458 amu.
But first you have to know that the isotopes of an element are those in which its atomic numbers (that is, the number of protons) are equal, but the number of neutrons is different.
Remember that protons and neutrons are found in the nucleus of the element.
The same chemical element may consist of different atoms, that is, different isotopes.
Then the atomic mass of an element is the weighted average mass of its natural isotopes. Therefore, the atomic mass of an element is not an integer.
In other words, the atomic masses of the chemical elements are usually calculated as the weighted average of the masses of the different isotopes of each element taking into account the relative abundance of each of them.
In this case, you know naturally occurring chlorine that is put in pools is 75.53 percent 35Cl (mass = 34.969 amu) and 24.47 percent 37CI (mass = 36.966 amu). Then, the atomic mass is calculated as:
0.7553×34.969 amu + 0.2447×36.966 amu= 35.458 amu
Finally, the average atomic mass is 35.5458 amu.
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