The hydrogen position in the periodic table :
Why Hydrogen placed at the first place in the periodic table ?
Hydrogen has the simplest atomic structure of all the elements. The atomic number of hydrogen is 1. The configuration of hydrogen is . With this configuration, it has to be placed in the 1st period in group I along with the alkali metals in the S – block of the periodic table.
Supporting points of hydrogen position placing in IA group :
Hydrogen resembles the alkali metals in its ability to form a hydrated unipositive ion. (aq).
Like alkali metals hydrogen also has a single electron in the outer shell .
It is quite reasonable to start the periodic table with an element having the least atomic number (Z = 1).
But the similarity ends here. Because of high ionization enthalpy, hydrogen does not easily give up its lone electron. On the other hand, alkali metals with low ionization energies do readily forms and ions.
Alkali metals preferably form ionic bonds.
Hydrogen has a great tendency to pair up its electron and form covalent bonds. Ex : Hcl, etc. In this respect, it resembles halogens (group VIIA).
Supporting points of Hydrogen position placing in VIIA group :
The halogens too, like hydrogen, are one electron short of the nearest inert gas configuration.
Halogens can easily form a negative () ion by gaining one electron. Hydrogen also can form a negative hybrid () ion, in specific instances.
The extremely small size of hydrogen atom with its moderately high electronegativity value explains negative ion formation.
It can form diatomic molecule like halogens.
Thus the position of hydrogen in the periodic table is now also uncertain. In view of this, it is difficult to assign any definite position to hydrogen. Sometimes it is placed in the IA group and sometimes with VIIA group.