The first periodic table was organized according to atomic mass, but the current periodic table is arranged according to atomic number. What is the difference between atomic mass and atomic number, and why is the atomic number now the basis for the periodic table?

Atomic mass is associated with the number of neutrons and protons that are present in a particular nucleus of an element. Atomic number is usually the number of protons present in an element's nucleus. It is the average weight of an element. It is the total number of nucleons in the atom's nucleus.

Assuming there were errors in atomic masses, Mendeleev placed certain elements not in order of increasing atomic mass so that they could fit into the proper groups (similar elements have similar properties) of his periodic table.

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