Importance Of Mosley's Law

Mosley's Law has been found to be very useful. The importance of this law is evident from the following points:

1. We have seen that this law has established that the atomic number is always numerically equal to the number of electrons revolving round the nucleus. According to this law it is the atomic number and not atomic weight on which the properties of the elements (both physical and chemical) depend. It thus says that the elements must be arranged in the periodic table in the increasing order of their atomic number and not atomic weight as originally enunciated by Mendeleef. Accordingly, Mosley's law has been used to place elements in their proper order in the periodic table in certain questionable cases, e.g. if the elements are placed according to their increasing atomic weight, K(Z=19) should come before Ar (Z=10) ; Ni (Z=28) should precede Co (Z=27) etc. But Mosley's law says that as per their atomic numbers their order should be reversed; i.e. K should be placed after Ar, Co should come before Ni etc. This fact is further supported by chemical properties.
2. Mosley's law has led to the discovery of many new elements. The elements like technetium (Z=43) have been discovered, promethium (Z=61), hafnium (Z=72), rhenium (Z=75) have been discovered on the basis of the gaps left by Mosley in Mosley's X-ray diagrams. These elements have been subsequently discovered.
3. This law has been helpful in determining the atomic number of rare-earths and has thus helped in fixing their position in the periodic table.