Hydrogen element has been placed at the top of group 1 above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals, both hydrogen as well as alkali metals have 1 valence electrons in each. Since hydrogen atom is very small in size many properties of Hydrogen a different from those of alkali metals. Therefore the alkali metal of group 1 hydrogen is never included hydrogen is not placed in any group and is treated as a very special element and placed alone at the head of the periodic table.
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Mendeleev created a periodic table by listing elements in a row in order of atomic weight and starting a new row the characteristics of element began to repeatThis resulted in arrangement where the element with similar properties occupies the same vertical columnMendeleev called these vertical columns as Groups and resultant horizontal rows as SeriesMendeleev success roots to his vision of classification of elements based on similarities in their empirical formulas and properties of compounds formed by themThis made him to occasionally ignore the order suggested by atomic weights to classify elements with similar properties togetherFor example Iodine with lower atomic weight than Tellurium was placed in Group 7 along with fluorine, chlorine and iodine because of its similarities with their propertiesMendeleev also left gaps in his periodic table for undiscovered elementsFor example he left gap under aluminum, silicon and Boron in the periodic table for yet to be discovered elementsMendeleev not only predicted the existence of these elements and called them Eka-Aluminium, Eka-silicon and Eka-Boron respectivelyMendeleev also described some of the general physical properties of above elementsLater these elements discovered and named as Gallium, Germanium and ScandiumHowever Mendeleev’s periodic table has certain drawbacksDrawbacks of Mendeleev’s Periodic Table:- a. Position of element hydrogen is not clear
- b. There is no place for isotopes of elements in his periodic table
- c. The placement of element having greater atomic mass before the elements having lower atomic mass could not be logically explained
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