About chemistry!

Isotopes are atoms that have the same number of protons but different numbers of neutrons.

The relative isotopic mass of an isotope is the relative mass (according to a scale of carbon-12 atom = 12.00000) of an atom of this isotope.

The relative atomic mass of an element is the weighted average of the relative isotopic masses (according to a scale of carbon-12 atom = 12.00000) of all isotopes of this element.


For example, chlorine exists in two isotopic forms Cl-35 and Cl-37 with relative abundance 75% and 25% respectively.

Relative isotopic mass of Cl-35 = 35

Relative isotopic mass of Cl-37 = 37

Relative atomic mass of Cl
= weighted average of relative isotopic masses of Cl-35 and Cl-37
= (35x75 + 37x25)/100
= 35.5
=