The first classification of elements was s metals and non-metals. It served only limited purpose. After atomic masses old term, atomic weight of elements had been determined, it was thought to be their most fundamental property and attempts were made to correlate it to their other properties. Doberman grouped elements into triads. The atomic mass and properties of the middle element were mean of the other two. He could group only a few elements into triads.

For example i Li, Na and K ii Ca, Sr and Ba iii Cl, Br and I. New land tried to see the periodicity of properties and stated his law of octaves that, “When elements are arranged in the increasing order of their atomic weights every eighth element has properties similar to the first”. He could arrange elements up to calcium only out of more than sixty elements known then. Mendeleev observed 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”. Mendeleev gave 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. 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. Number of valence electrons, ionization energy, electron affinity and electro negativity increase while metallic character and atomic and ionic radii decrease in a period from left to right.

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