Abstract:

Atomic weight is one of the fundamental scientific concepts. "The IUPAC Commission on the Isotopic Abundances and Atomic Weights" revises and releases atomic weights of the elements every two years. Since 2009, IUPAC announced that the atomic weights of some elements are no longer constants of nature, and the standard atomic weights of these elements span an interval. For the convenience of users, a single value is also provided for these elements, which is called "conventional atomic weight". What is the standard atomic weight? What is a conventional atomic weight? Why do the atomic weights of some elements span an interval? According to the characteristics of atomic weight values, how many classes can the elements be divided into? After introducing the characteristics of atomic weights in the latest periodic table of the elements, this article briefly deals with the history of atomic weight determination, the development of the standard of the unified atomic mass unit, and then discusses the revision and changes of atomic weights.

Key words: Atomic weight, Standard atomic weight, Conventional atomic weight, Isotope abundance, Atomic weight interval, Atomic mass unit