Hydrogen is said to be an analogous element when its position in the periodic table is considered. This is because it shares its properties both with alkali metals as well as with halogens in the field of sharing or gaining electrons or in other words it has its electronic configuration similar to these. That is why its position in the periodic table is not justified.
Hydrogen (1s1) shares the similar electronic configuration with halogens where both of them require to gain only one electron to obtain the stable state configuration. Eg:
F : 1s 2 2s 2 2p 5
Cl : 1s 2 2s 3 2p 6 3s 2 3p 5
So, we can see that according to this electronic configuration, both hydrogen and halogens require to gain only one electron to acquire the stable duplet(for hydrogen) and octet(for halogens) states or a uni-negative ion.
Also the electronic configuration of hydrogen (1s1) is similar to that of alkali metals (ns1) which shows that only one electron is present in their valence shell. So if they lose that single electron, a uni-positive ion would be formed.
Also, the size of a halogen atom does not remain the same all throughout the periodic table. As we go down a group, in a periodic table, more number of electrons are added to the last shell. Thus the nuclear charge cannot reach to the last shells due to which, atomic radius increases.
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Justify the positNCERT - Chemistry Part-II