Q.Justify the position of hydrogen in the periodic table on the basis of its electronic configuration.
Hydrogen has the electronic configuration — a single electron in its only shell.
Resemblance to alkali metals (Group 1, ):
- Like alkali metals, hydrogen has one electron in its valence shell.
- It can lose this electron to form a unipositive ion H⁺, just as alkali metals form M⁺.
- It exhibits an oxidation state of +1 in most of its compounds, exactly like the alkali metals.
- It combines with electronegative elements to form compounds analogous to alkali metal compounds (e.g., HCl and NaCl; H2O and Na2O type behaviour).
Resemblance to halogens (Group 17, ):
- Like halogens, hydrogen is only one electron short of the nearest noble-gas (helium, duplet) configuration.
- It can gain one electron to form a mononegative hydride ion H⁻, just as halogens form X⁻.
- It exists as a diatomic molecule H2, held together by a single covalent bond, exactly as halogens exist as X2 (Cl2, Br2, etc.).
- Its ionisation enthalpy, while not identical, is of a comparable high order to those of halogens (rather than the low values typical of alkali metals).
Conclusion: Because hydrogen shows genuine similarities to both groups but is not a full member of either (its ionisation enthalpy is far higher than alkali metals', and it lacks the strong electronegativity/multiple lone pairs of halogens), it occupies an anomalous, singular position in the periodic table and is frequently shown placed in both Group 1 and Group 17.
Hydrogen's single 1s¹ electron lets it lose an electron (like alkali metals, forming H⁺) or gain an electron (like halogens, forming H⁻), so its position is justified by dual resemblance, and it is placed separately/in both groups.
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