Which identifies an element, the atomic number or atomic weight?

1 Answer
Aug 10, 2017


Each element has its own unique atomic number, which is the number of protons in the nuclei of its atoms. The number of protons is always a whole number.

Atomic mass and atomic weight are weighted averages of the isotopes in a sample of an element, and because its an average neither atomic mass nor atomic weight are whole numbers; they are always fractions.