Dmitri Mendeleev, the Russian chemist credited with creating the first widely accepted periodic table of elements in 1869, did not use the concept of atomic number to arrange the elements because it was not yet discovered or understood during his time.
When Mendeleev developed his periodic table, the concept of atomic number had not been established. At that time, the understanding of atomic structure was limited, and the nature of atomic particles, such as protons, neutrons, and electrons, was not well-known.
Mendeleev organized the elements based on their chemical and physical properties and their atomic masses. He noticed that when the elements were arranged in order of increasing atomic mass, their properties repeated periodically, which led to the development of the periodic table. Mendeleev left gaps in his table for elements that were not yet discovered but predicted their existence and properties based on the patterns he observed.
It was only later, in the early 20th century, that Henry Moseley's experiments with X-rays provided evidence for the concept of atomic number. Moseley discovered that the elements' properties were better aligned with their atomic numbers (which represent the number of protons in an atom's nucleus) rather than their atomic masses. This led to the modern understanding that the periodic table should be organized based on atomic number.
So, to summarize, Mendeleev did not use the atomic number because it was not known or understood during his time. He arranged the elements based on atomic mass and observed patterns in their chemical and physical properties. The concept of atomic number and its significance in organizing the periodic table emerged later with the work of Henry Moseley.