Main defects of Mendeleev’s periodic table were i position of isotopes, ii anomalous pairs of elements like Ar and K and iii grouping of dissimilar elements and separation of similar elements. Moseley discovered that atomic number and not atomic mass is the most fundamental property of elements. In the light of this the periodic law was modified to “The chemical and physical properties of elements are a periodic function of their atomic numbers”. This is the modern periodic table. Modern periodic table is based upon atomic number. Its long form has been accepted by IUPAC. It has seven periods 1 to 7 and 18 groups 1 to 18. It is free of main defects of Mendeleev’s periodic table.

Elements belonging to same group have same number of valence electrons and thus show same valence and similar chemical properties. Arrangement of elements in the periodic table shows periodicity. Atomic and ionic radii and metallic character increase while ionization energy, electron affinity and electro negativity decrease in a group from top to bottom.

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