He noticed that when elements were arranged in order of increasing atomic mass, there were recurring patterns in their properties. This led him to develop the periodic table, which he published in 1869.
It's important to note that while atomic mass was the primary basis for Mendeleev's arrangement, it wasn't the only factor. He also recognized that certain elements needed to be placed out of order based on their chemical properties. This demonstrated his understanding of the importance of chemical periodicity.
Later, the modern periodic table was developed, where elements are arranged in order of increasing atomic number, which is a more accurate reflection of the underlying structure of atoms.
