![]() ![]() It was not possible, therefore, to predict how many elements could be discovered between two heavy elements. The rise in atomic mass does not appear to be uniform when elements are arranged in an increasing order of atomic masses.A challenge was posed in placing isotopes in Mendeleev’s periodic table as isotopes have the same chemical properties but different atomic masses. Isotopes were discovered long time after Mendeleev put forth the periodic table.As a result there was an ambiguity regarding their sequence in Mendeleev’s periodic table. The elements cobalt (Co) and nickel (Ni) have the same whole number atomic mass.When noble gases such as helium, neon and argon were discovered, Mendeleev create the ‘zero group’ without disturbing the original periodic table in which the noble gases were placed very well.Due to this success all were convinced about the importance of Mendeleev's periodic table.The properties of these elements matched well with those predicted by Mendeleev. Later on these elements were discovered subsequently and were named as scandium (Sc), gallium (Ga) and germanium (Ge) respectively.Three of these unknown elements were given the names eka-boron, eka-aluminium and eka-silicon from the known neighbours and their atomic masses were indicated as 44, 68 and 72, respectively.Mendeleev had kept some vacant places in the periodic table for elements that were yet to be discovered.For example, the previously determined atomic mass of beryllium, 14.09, was changed to the correct value 9-4, and beryllium was placed before boron.To give the proper place in the periodic table, atomic masses of some elements were revised in accordance with their properties.The vertical columns in the periodic table are called groups while the horizontal rows are called periods. Mendeleev’s periodic law: The elements with physical and chemical properties are a periodic function of their atomic masses. ![]() Mendeleev found that the elements with similar physical and chemical properties repeat after a definite interval.Then he transformed this into the periodic table of elements according to their physical and chemical properties.He arranged 63 elements known at that time in an increasing order of their atomic masses.Mendeleev’s Periodic table: The most important step in the classification of elements in Mendeleev’s periodic table is the fundamental property of elements, namely, the atomic mass, as standard. The periodic table did not include inert gases because they were not discovered then. ![]()
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