Mendeleev’s periodic law stated that the properties of elements are a periodic function of their atomic masses. By arranging elements in order of increasing atomic mass, Mendeleev noticed that elements with similar properties occurred at regular intervals. He also predicted the existence and properties of elements that had not yet been discovered, based on gaps in his table. This was a major breakthrough, as it demonstrated that properties of elements are related in a periodic way. Later, the discovery of atomic number by Moseley led to the modern periodic table, where elements are arranged by increasing atomic number instead of atomic mass. This corrected some inconsistencies in Mendeleev’s original arrangement and led to a more accurate prediction of element properties.