Answer:
Hydrogen is the first element of the periodic table & often called protium. Its electronic configuration is [1s1]. Due to the presence of only one electron in its 1s shell, hydrogen exhibits a dual behaviour, i.e., it resembles both alkali metals and halogens. As a result it is placed at the top of the alkali metals in group 1 & also along the halogens in group 17 since just like halogens,it also requires any one electron to have the configuration of the nearest noble gas element..The dual nature of hydrogen can be justified by following points:
Resemblance with alkali metals:
1. Like alkali metals, hydrogen contains one valence electron in its valence shell.
H : 1s1
Li : 1s2 2s1
Na : 1s2 2s2 2p6 3s1
Hence, it can lose one electron to form a unipositive ion.
2 Both hydrogen & alkali metals form monovalent cations by losing the electrons in the valence shell,showing hydrogens electropositive character similar to alkali metals.
H --------> H+ + e- , Na ---------->Na+ + e-
3 In their compounds both hydrogen & alkali metals show +1 oxidation states. Eg Hcl & Nacl
4 Both hydrogen & alkali metals are electropositive in nature,they have affinity for the electronegative elements & readily combine with them & form oxides, halides, and sulphides.
Resemblance with halogens:
1. Both hydrogen and halogens require one electron to complete their octets.
H : 1s1
F : 1s2 2s2 2p5
Cl : 1s2 2s2 2p6 3s2 3p5
Hence, hydrogen can gain one electron to form a uninegative ion.
2. Like halogens, it forms a diatomic molecule and several covalent compounds.
Though hydrogen shows some similarity with both alkali metals and halogens, it differs from them on some grounds. Unlike alkali metals, hydrogen does not possess metallic characteristics. On the other hand, it possesses a high ionization enthalpy. Also, it is less reactive than halogens.
Owing to these reasons, hydrogen cannot be placed with alkali metals (group I) or with halogens (group VII). In addition, it was also established that H+ ions cannot exist freely as they are extremely small. H+ ions are always associated with other atoms or molecules. Hence, hydrogen is best placed separately in the periodic table.
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