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Need And Genesis Of Classification

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Need And Genesis Of Classification - Lesson Summary

Periodic table helps classify all the known elements in rows and columns such that the periodicity in their properties becomes apparent. The periodic law defines this periodicity.

The classification of elements in a periodic table helps to predict the properties of unknown elements.

Johann Dobereiner was the first scientist to observe trends in the properties of elements in 1817 and stated the law of triads based on observations.

Element Atomic
Weight Element Atomic
Weight Element Atomic
Weight    Li    7    Ca    40    Cl    35.5    Na    23    Sr    88    Br    80    K    39    Ba    137    I    127

But Dobereiner's Law of Triads seemed to true only for a few of the known elements and hence it was dismissed.

The first periodic table that applied to all the known elements was formulated by A.E.B. de Chancourtois in 1862. Chancourtois's cylindrical periodic table was based on atomic weights of elements and displayed the periodic recurrence in their properties. But, chemists did not give his discovery its due attention because it was written in terms of geology.

John Alexander Newlands gave the Law of Octaves which states that when the elements are arranged in the increasing order of their atomic weights, the properties of every eighth element are similar to that of the first element.

Element Li Be B C N O F Atomic Weight 7 9 11 12 14 16 19 Element Na Mg Al Si P S Cl Atomic Weight 23 24 27 29 31 32 35.5 Element K Ca

But, this Law was not found to be true for new elements found after Calcium.

Lothar Meyer and Dmitri Mendeleev independently proposed the Periodic Law, in 1869.

Lothar Meyer observed a periodically repeating pattern of chemical and physical properties against their atomic weight, while analysing atomic volume, melting point and boiling point of elements. Based on this he proposes proposed a classification of elements.

I II III IV V VI VII VIII IX B = 11.0 Al = 27.3 Ln = 113.4 Ti = 202.7 C = 11.97 Si = 28 Sn = 117.8 Pb = 206.4 Ti = 48 Zr = 89.7 N = 14.01 P = 30.9 As = 74.9 Sb = 122.1 Bi = 207.5 V = 51.2 nb = 93.7 Ta = 182.2 O = 15.96 31.98 Se = 78 Te = 128 Cr = 52.4 Mo = 95.6 W = 183.5 F = 19.1 Cl = 35.38 Br = 79.75 I = 126.5 Mn = 54.8 Ru = 103.5 Os = 198.6 Fe = 55.9 Rh = 104.1 Ir = 196.7 Co = Ni = 58.6 Pd = 106.2 Pt = 196.7 Li = 7.01 Na = 22.99 K = 39.04 Rb = 85.2 Cs = 132.7 Cu = 63.3 Ag = 107.66 Au = 196.2 Be = 93.3 Mg = 23.9 Ca = 39.9 Sr = 87.0 Ba = 136.8 Zn = 64.9 Cd = 111.6 Hg = 199.8

                                     Lothar Mever's Studv on Classification of Elements

Unfortunately, his work was published after the work of Dmitri Mendeleev, hence Mendeleev is considered the father of the Modern Periodic Table.

In 1869, Russian chemist Dmitri Mendeleev published a periodic table and corresponding periodic law of 63 elements known at that time.

According to Mendeleev's periodic law, the physical and chemical properties of the elements are a periodic function of their atomic weights

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