Q.What is the basic difference in approach between Mendeleev's Periodic Law and Modern Periodic Law?

Henry Moseley (1913) studied the characteristic X-rays emitted by elements bombarded with high-energy electrons and found a strict linear relationship between the square root of the emitted X-ray frequency $\nu$ and the element's atomic number $Z$:

with $a$ and $b$ constants common to all elements. This let atomic number be measured directly and unambiguously, and it is atomic number -- not atomic weight -- that Moseley's results showed the elements' properties are truly periodic functions of. This is the modern periodic law: the physical and chemical properties of the elements are periodic functions of their atomic numbers. Elements are arranged in the periodic table by increasing atomic number, and this ordering is what resolves the …