Atomic number
Atomic mass was used to organize the earliest periodic table.
number of protons
The properties of elements are found to be periodic when arranged according to atomic number. Hence atomic number is considered as base for arranging the elements.
Atomic weight
Atomic number was not used by Mendeleev to organize his periodic table. He organized the elements based on their atomic mass and properties, leaving gaps for undiscovered elements that were later filled.
He organised the elements in the increasing order of the atomic mass.
atomic number
No. of electrons especially in outer most shell
The elements are organised in the periodic table of elements by order of ascending atomic number. They are arranged in groups and periods to demonstrate the gradual trends that are characteristic of certain groups of elements.
This is the number of protons.
This is the number of protons.
Atomic number on the basis of Modern periodic law as on the view of Henry Mosely