Note 2 Relative atomic mass is the ratio of the average atomic mass of an element to the atomic mass of nuclide12 c112. The relative atomic mass used to be called atomic mass. The relative atomic mass is dimensionless and is usually represented by the symbol Ar. Ar (Cl) is 35.4527, indicating that the relative atomic mass of chlorine is 35.4527. The "average atomic mass" in the definition is for multi-elements. The average atomic mass is the sum of the products of atomic mass and abundance of various nuclides of this element. For example,
1H 1.007825u 99.985%
2H
The average atomic mass of hydrogen is 1.007825 U× 99.985%.
2.0 140u×0.0 15%
1.00794u
The relative atomic mass is a relative value, and the benchmark for comparison has been changed several times in history. 1803 Dalton published the first atomic scale based on H 1 atomic weight. 1826, Pachilius proposed that the atomic weight of O should be1100. Later, based on the atomic weight of O116, it was used for a long time in the history of chemistry. 1929 found that there are three isotopes of oxygen (16O, 17O and 18O), so it seems inappropriate to take atomic weight 0 as the benchmark. At this time, the physical circle is based on116 of 160. 1959 Matouch put forward a new standard based on112 of12, and the international union of pure and applied physics of 1960 and the international union of pure and applied chemistry of 196 1 agreed successively.