The terms "atomic mass" and "atomic weight" are related but have slightly different meanings.
Atomic Mass: Atomic mass refers to the mass of an individual atom of an element. It is usually expressed in atomic mass units (amu) or unified atomic mass units (u). The atomic mass of an atom is determined by the total number of protons and neutrons in its nucleus. Atomic mass values are generally whole numbers or decimal fractions close to whole numbers because they are calculated based on the average mass of isotopes (atoms with different numbers of neutrons) of an element found in nature.
Atomic Weight: Atomic weight, also known as atomic mass or relative atomic mass, is the weighted average mass of all the isotopes of an element, taking into account their abundance in nature. It is a weighted average because it considers the relative proportions of different isotopes of an element. Atomic weight is also expressed in atomic mass units (amu) or unified atomic mass units (u). Atomic weight values are typically decimal numbers.
As for Dmitri Mendeleev, he arranged the elements in his periodic table based on atomic mass, not atomic weight. He observed that when elements were arranged in order of increasing atomic mass, there was a periodic pattern of properties. Mendeleev's periodic table formed the basis for the modern periodic table, which is arranged in order of atomic number (the number of protons in the nucleus) rather than atomic mass. However, the modern periodic table still reflects the periodicity of properties based on the underlying trends in atomic mass.