Hydrogen is the first elements of the periodic table. Its electronicn configuration is `[1s^(-1)]` Dueo 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. ltbr Resembleance wit alkali metals:
1:Like alkali metals, hydrogen contains one valence electron in tis valency shell.
`H=1s^(-1)`
`Li:[He]2s^(-1)`
`Na:[Ne]3s^(-1)`
Hence, it can lose one electron to form a unipositive ion.
2. Like alkali metals, hydrogen combines with electronegative elements to form oxides, haldies, and sulphides.
Resemblance with halogens: 1. Both hydrogen an dhalogens require one electrons to complete their octets.
`H:1s^(-1)`
`F:1s^(2)2s^(2) 2p^(5)`
`Cl:1s^(2)2s^(2)2p^(6)3s^(2)3p^(5)`
Hence, hydrogen can give one electron to form a uninegative ion.
2. Like halogen, 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 alkalli metals, hydrogen does not possess metallic characteristics. On the other hand,it possesses a high ionization enthalpy. Also, it is les reactive than halogens. Owing to these reasons, hydrogen cannot be placed with alkali metals (group I ) or with halogns (group VII). In addition, it was also established that `H^(+)` ions cannot exist freely as they aer extremely small. `H^(+)` ions are always associated with other atoms or moleculess, Hence, hydrogen is best separately in the periodic table.
