correlation between atomic masses and other properties and stated his periodic law as, “The chemical and physical properties of elements are a periodic function of their atomic weights”. the first periodic table which is named after him which included all the known elements. It consists of seven horizontal rows called periods and numbered from 1 to 7. It has nine vertical columns called groups and numbered from zero to VIII.

Main achievements of Mendeleev’s periodic table were i inclusion of all the known elements and ii prediction of new elements. 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.

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