In chemistry you can have different versions of the same element which are known as Isotopes
The relative abundance of an isotope is the percentage of atoms with a specific atomic mass found in a naturally occurring sample of an element.
Isotope Notation
35 Cl 17
35 - Represents mass (protons + neutrons) Cl - Element symbol 17 - # of protons
Average atomic mass
They take the the % of how much of each element exists the mass of that specific isotope and add it up to find the average atomic mass Average atomic mass