He published his periodic table in 1869, arranging the elements by atomic weight, which was the best understood property of elements at the time. He also recognized that there were gaps in his table, and he predicted the existence of undiscovered elements based on their expected properties.
Later, Henry Moseley refined Mendeleev's work by using X-ray spectroscopy to determine the atomic number of each element. Moseley's work confirmed that atomic number, not atomic weight, was the fundamental basis for the periodic table.
