Position of hydrogen in the periodic tablePosition of an element in periodic table depends upon its electronic configuration and properties. Hydrogen resembles the elements of group I–A, IV–A and VII–A in some respects.
Properties of hydrogen do not completely match any one of the above–mentioned groups. That is why position of hydrogen is still undecided. But its position is usually shown above the alkali metals due to the presence of one electron in its valence shell as alkali metals.
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Hydrogen has no perfectly suitable position in the periodic table. Because of its single valence electron and common +1 oxidation state, some think that it almost fits in Group 1A (1). However, unlike the alkali metals, hydrogen shares its electron with non‐metals rather than loosing it to them. Moreover, like a non‐metal, it has a relatively high ionization energy (IE = 1311kJ/mol) much higher than that of lithium (520kJ/mol), whose IE is the highest in Group 1A. Hydrogen has a much higher electronegativity of 2.1 with the electronegativity of Li being 1.0.
Others think hydrogen almost fits in Group 7A (17). Like the halogens, it occurs as diatomic molecules and fills its outer level either by electron sharing or by gaining one electron from a metal to form a monoatomic anion, the hydride ion H−, with charge of −1. However, hydrogen has a lower electronegativity value of 2.1 which is less than any of the halogens which range from 2.2 to 4.0 and lacks their three valence electron pairs. Moreover, the H− ion is rare and reactive, whereas halide ions are common and stable.
And still others think that, because of a half‐filled valence level and similarities in ionization energy, electron affinity, electronegativity and bond energies, hydrogen fits best in Group 4A (14). Hydrogen's unique behavior is attributable to its tiny size. Hydrogen has a high ionization energy because its electron is very close to the nucleus, with no inner electrons to shield it from the positive charge. It has a low electronegativity (for a non‐metal) because it has only one proton to attract bonding electrons.