Coupon Accepted Successfully!


Position of Hydrogen in the Periodic Table

Hydrogen is the lightest, smallest and the first element of the periodic table. Its electronic configuration (1s1) is the simplest of all the elements. It occupies a unique position in the periodic table. In its properties, it behaves like alkali metals as well as halogens. In the periodic table, it is placed at the top of the alkali metals.

Hydrogen can lose its only electron to form a hydrogen ion (H+) in which case it resembles alkali metals (M) that yield M+ ions. It can also gain one electron to form the hydride ion (H-) similar to halogens (X) forming halide ions (X-). Thus hydrogen is unique and anomalous in being similar to two different groups of elements. Let us now examine these similarities in detail.

Similarities to Halogens

Hydrogen is one electron short, in its orbit, of the stable configuration of the noble gas helium. In halogens, too, the outermost orbital contains one electron less than the maximum number of electrons that can be accommodated. Hydrogen can, thus, gain one electron to form H- ion as the halogens do, forming X- ions.

H + e- H- (Hydride ion)

X + e- X- (Halide ion)

                                          Electronic Configurations of Hydrogen and Halogens


Electronic configuration




2, 7


2, 8, 7


2, 8, 18, 7


2, 8, 18, 18, 7

With electropositive alkali and alkaline earth metals (Li, Na, Ca, etc.), hydrogen forms compounds like Li+H-, Na+H-, Ca2+(H-)2, etc. similar to halogens forming Li+Cl-, Na+Cl-, Ca2+(Cl-)2, etc. On being electrolysed, these hydrides yield hydrogen or halogen at the anode showing the similarity between the two.

The ionization energies of hydrogen and halogens are comparable while those of alkali metals are very low. Hydrogen is similar to halogens in being a nonmetal, a bad conductor of heat and electricity and diatomic (H2). The diatomic H2 molecule, we shall refer to as dihydrogen in order to distinguish it from the hydrogen atom.

It forms covalent compounds like those formed by halogens. For example, hydrogen forms CH4, SiH4, etc. similar to chlorine forming CCl4, SiCl4, etc. The hydrogen compounds, NH3 and PH3 are similar to NCl3 and PCl3 (halogen compounds).

First Ionization Energies of Hydrogen, Halogens and Alkali


In covalent compounds, hydrogen and halogens are interchangeable showing their similarity. For example, in methane, hydrogen can be successively replaced by chlorine. Also, in methyl chloride, chlorine can be replaced by hydrogen.

Hydrogen constitutes 0.9% by mass of the earth's crust and is placed ninth in order of abundance. It is present in water (H2O), hydrocarbons, organic matter, coal petroleum, clay, etc.

The planets Jupiter and Saturn contain mostly hydrogen. The sun provides enormous energy because of the fusion of hydrogen nuclei taking place there.

Test Your Skills Now!
Take a Quiz now
Reviewer Name