Modern Periodic Law
 Henry Moseley, a British Physicist, 1912
This law states that the physical and chemical properties of elements are periodic function of their atomic number. It means if the elements are arranged in order of increasing atomic number, the elements with similar properties are repeated after regular intervals.
Main features of modern periodic table
 It has seven horizontal rows called periods.
  It has 18 vertical columns called groups.
  Elements are classified in different blocks on the basis of electronic configuration.
a) S – block elements
b) P – block elements
c) d – block elements
d) f  – block elements

 

Advantages of Modern periodic table
1. Modern periodic table is based on the most fundamental property, the atomic number of elements, while Mendeleev's periodic table is based upon the atomic masses of elements.
2. Modern periodic table explains clearly why elements in a group display similar properties. It also explains why elements of a group differ in properties from the elements of other groups.
3. Modern periodic table gives a satisfactory explanation about the chemical periodicity in the properties of elements. The periodicity in properties arises due to periodicity in electronic configurations of elements.
4. In Mendeleev's periodic table there are several anomalies, e.g. the position of isotopes, wrong order of atomic masses of some elements etc. In the long form of the periodic table, these anomalies have been removed.
5. In the long form of the periodic table, elements have been clearly separated as normal elements, transition elements and noble gases. Metals and nonmetals are also separated. But in Mendeleev's periodic table there is no such separation of different types of elements.
6. In the modern periodic table the subgroups A and B are clearly separate because the elements belonging to subgroup A differ in properties from those of elements belonging to subgroup B. In Mendeleev's periodic table the two subgroups are kept together.


Limitations of modern periodic table
1. The position of Hydrogen is still undecided.
2. Group VIII B consists of three columns instead of one.
3. Like Mendeleev's table it fails to accommodate the Lanthanides and Actinides in the main body of table.
4. Some elements which resemble in chemical properties are placed in separate groups like Li in IA and Mg in IIA . They show similarities in properties but are placed in separate groups.