(a) Due to discovery of different elements, it was difficult to organize the information and properties of the elements, so there was a strong need to find some pattern in the properties of different elements so that they can be grouped together. That’s why we classify elements.
(b) Mendeleev used atomic mass and similarities in chemical properties to classify the elements.
(c) Mendeleev predicted existence of some elements in the periodic table which were not discovered at that time, so he left place for those elements in the periodic table.
(d) Nobel gases are inert and are present in extremely low concentration in the atmosphere.
That’s why Mendeleev didn’t placed Nobel gases in the periodic table. When they were discovered, they were placed in a different group so that they don’t create any difference in the periodic table.
(d) Since Mendeleev used atomic mass and similar properties as a criteria to classify elements but Cl-35 and Cl-37 have different atomic masses, so they should be placed at different places but the two isotopes have similar chemical properties, so they should be placed in same slot. This was a contradiction and therefore, the position of the isotopes of an element was not clear in the Mendeleev’s periodic table.
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