Dmitri Mendeleev, the Russian chemist credited with the development of the periodic table, did not directly measure atomic masses himself. Instead, he relied on existing experimental data and observations of the chemical and physical properties of elements to arrange them in a systematic manner.
At the time of Mendeleev's work in the late 19th century, the concept of atomic mass was not yet fully understood, and accurate measurements of atomic masses were challenging. However, scientists had made progress in determining relative atomic masses by comparing the masses of elements to a standard reference element.
Mendeleev used the available atomic mass data, although they were less precise than what we have today, to organize elements in a logical manner. He observed that certain properties of elements repeated periodically with increasing atomic mass. By arranging elements according to their properties and patterns, he was able to identify gaps in the periodic table where he predicted the existence of undiscovered elements and even made predictions about their properties.
While Mendeleev's periodic table was primarily based on atomic mass, he also took into account the chemical and physical properties of elements to create a comprehensive and predictive arrangement. As scientific understanding and technology progressed, more accurate measurements of atomic masses were obtained, further refining and confirming Mendeleev's periodic table.