Topic to covered

`=>` General introduction
`=>` Position of hydrogen in periodic table


`color{green}(★)` Hydrogen has the simplest atomic structure among all the elements around us in Nature consisting of only one proton and one electron in atomic form.

`color{green}(★)` However, in elemental form it exists as a diatomic (`color{red}(H_2)`) molecule and is called dihydrogen. It forms more compounds than any other element. ?

`color{green}(★)` In fact, hydrogen is of great industrial importance as you will learn in this unit.


`color{green}(⧫)` 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.
`color{maroon}(★)` `color{maroon} (" REASON")`

`color{green}(★)` The elements in the periodic table are arranged according to their electronic configurations.

`color{green}(★)` Hydrogen has electronic configuration `color{red}(1s^1)`. On one hand, its electronic configuration is similar to the outer electronic configuration (`color{red}(ns^1)`) of alkali metals , which belong to the first group of the periodic table.

`color{green}(★)` On the other hand, like halogens (with `color{red}(ns^2np^5)` configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (`color{red}(1s^2)`).

`color{green}(★)` 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.

`color{green}(★)` 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.

`color{green}(★)` In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens, `color{red}(Δ_i H)` of `color{red}(Li)` is `color{red}(520 kJ mol^(–1), F)` is `color{red}(1680 kJ mol^(–1))` and that of `color{red}(H)` is `color{red}(1312 kJ mol^(–1))`.

`color{green}(★)` 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.

`color{green}(⧫)` 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?

`color{green}(⧫)` Loss of the electron from hydrogen atom results in nucleus (`color{red}(H^+)`) of `color{red}(~1.5 xx 10^(–3))` pm size. This is extremely small as compared to normal atomic and ionic sizes of `color{red}(50)` to `color{red}(200)` pm. As a consequence, `color{red}(H^+)` does not exist freely and is always associated with other atoms or molecules.

`color{green}(⧫)` Thus, it is unique in behaviour and is, therefore, best placed separately in the periodic table (Unit 3).