Early periodic tables did not use atomic numbers because the concept of atomic number was not yet discovered or understood. The development of the modern periodic table is a result of the gradual understanding of atomic structure and the arrangement of elements.
In the early days of chemistry, elements were primarily classified based on their chemical and physical properties. Chemists recognized that certain elements exhibited similar properties and could be grouped together. The first attempts at organizing elements were based on atomic masses, with elements arranged in ascending order of their relative atomic masses.
Dmitri Mendeleev, a Russian chemist, is credited with the development of the first widely accepted periodic table in 1869. He arranged the elements in order of increasing atomic masses and observed that elements with similar properties recurred periodically. Mendeleev left gaps in his table for elements that were yet to be discovered, and he predicted their properties based on the patterns he observed.
It was only later, in the early 20th century, that Henry Moseley, an English physicist, discovered the concept of atomic number. Moseley's experiments with X-ray spectroscopy revealed a direct relationship between an element's atomic number (the number of protons in its nucleus) and its X-ray emission spectrum. This discovery established the concept of atomic number as the fundamental property of an element.
With the realization that atomic number was a more fundamental characteristic than atomic mass, the modern periodic table was revised to arrange elements in order of increasing atomic number rather than atomic mass. This change provided a more accurate representation of the periodic trends and allowed for a better understanding of the underlying structure of atoms and their electron configurations.