Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past. As you know by now that the elements in the periodic table are arranged according to their electronic configurations.
Hydrogen has electronic configuration 1s1 . On one hand, its electronic configuration is similar to the outer electronic configuration (ns1 ) of alkali metals , which belong to the first group of the periodic table.
On the other hand, like halogens (with ns2 np5 configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (1s2 ).
Hydrogen, therefore, has resemblance to alkali metals, which lose one electron to form unipositive ions, as well as with halogens, which gain one electron to form uninegative ion. Like alkali metals, hydrogen forms oxides, halides and sulphides.
However, unlike alkali metals, it has a very high ionization enthalpy and does not possess metallic characteristics under normal conditions.
In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens, ∆iH of Li is 520 kJ mol –1 , F is 1680 kJ mol–1 and that of H is 1312 kJ mol –1 .
Like halogens, it forms a diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds. However, in terms of reactivity, it is very low as compared to halogens.
Inspite of the fact that hydrogen, to a certain extent resembles both with alkali metals and halogens, it differs from them as well.
Now the pertinent question arises as where should it be placed in the periodic table? Loss of the electron from hydrogen atom results in nucleus (H+ ) of ~1.5×10–3 pm size.
This is extremely small as compared to normal atomic and ionic sizes of 50 to 200pm. As a consequence, H + does not exist freely and is always associated with other atoms or molecules.
Thus, it is unique in behaviour and is, therefore, best placed separately in the periodic table (Unit 3).