How does abundance contribute to the average atomic mass of an element?

1 Answer
Feb 12, 2018


Percentage abundance of isotopes determines relative atomic mass.


Chlorine has #A_r# value of 35.5 as there are two isotopes, Cl-35 and Cl-37. These do not occur in equal proportions. In every 100 atoms, there are 75 of Cl-35 and 25 of Cl-37. Therefore, the #A_r# value is calculated as

#((35 xx 75) + (37 xx 25))/100 = 35.5#