which isotope is used as relative scale to find atomic mass
Relative atomic mass Relative atomic mass is determined by the average atomic mass, or the weighted mean of the atomic masses of all the atoms of a particular chemical element found in a particular sample, which is then compared to the atomic mass of carbon-12.[5] This comparison is the quotient of the two weights, which makes the value dimensionless (no unit appended). This quotient also explains the word relative: the sample mass value is made relative to carbon-12.
Isotope Isotopes are atoms of the same element having the same numbers of protons (atomic number), but different numbers of neutrons. They have same chemical properties due to the same electron configuration, but different physical properties.[citation needed]