It is a colourless, tasteless and odourless gas. It is slightly soluble in water. It is highly combustible. The Physical constants of atomic hydrogen are,

Atomic radius (pm) : `37` ;

Ionic radius of `H^-` ion (pm) : `210`;

Ionisation energy (`kJ mol^-1`) : `1312`;

Electron affinity (`kJmol^-1`) : `72.8` ;

Electronegativity : `2.1.`

Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated above `2000` `K`,

`H_2 overset(2000 K) (->) H + H`.

Its bond dissociation energy is very high, `H_2 -> H + H` ; `Delta = 435.9 kJmol^-1`.

Due to its high bond dissociation energy, it is not very reactive. However, it combines with many elements or compounds.

(i) `text(Action with metals)` : To form corresponding hydrides.

`2 Na + H_2 oversettext(Heat) -> 2NaH`; `Ca + H_2 oversettext(Heat) -> Ca H_2`

With transition metals (elements of `d`- block) such as `Pd`, `Ni`, `Pt` etc. dihydrogen forms interstitial hydrides in which the small molecules of dihydrogen occupy the interstitial sites in the crystal lattices of these hydrides. As a result of formation of interstitial hydrides, these metals adsorb large volume of hydrogen on their surface. This property of adsorption of a gas by a metal is called `text(occlusion)`. The occluded hydrogen can be liberated from the metals by strong heating.

(ii) `text(Reaction with Non-metals)` :

`2H_2 + O_2 overset (970 K) (->) 2H_2O`;

`N_2 + 3H_2 undersettext(750 K, Pressure) overset (Fe-Mo) -> 2 NH_3`

`H_2 + F_2 oversettext(dark) (-> ) 2 HF`;

`H_2 +Cl_2 undersettext(Sunlight) oversetext(673 K , Pressure) -> 2HCl`

`H_2 + Br_2 -> 2HBr`; `H_2 + I_2 underset (Pt) overset (673 K) (->) 2 HI`

The reactivity of halogen towards dihydrogen decreases as `F_2 > Cl_2 > Br_2 > I_2`

As a result, `F_2` reacts in dark, `Cl_2` in the presence of sunlight, `Br_2` reacts only upon heating while the reaction with `I_2` occurs in the presence of a catalyst.

(iii) `text(Reaction with unsaturated hydrocarbons)` :

`H_2`, reacts with unsaturated hydrocarbons such as ethylene and acetylene to give saturated hydrocarbons.

`undersettext(Ethylene) (H_2C =CH_2) + H_2 underset(473 K) overset (Ni or Pt or Pd) (->) undersettext(Ethane ) (CH_3 - CH_3 )`;

`undersettext(Acetylene) (HC-=CH +2H_2) underset (473 K) overset (Ni or Pt or Pd) (->) undersettext(Ethane ) (CH_3 - CH_3)`

This reaction is used in the hydrogenation or hardening of oils . The vegetable oils such as groundnut oil or cotton-seed oil are unsaturated in nature because they contain at least one double bond in their molecules. Dihydrogen is passed through the oils at about `473 K` in the presence of catalyst to form solid fats. The vegetable ghee such as Dalda, Rath, etc. are usually prepared by this process.

`undersettext(Liquid) [text(Vegetable oil)] + H_2 underset (473 K) overset (Ni) (->) undersettext(Solid) [text(Fat)] `

(i) As a reducing agent.

(ii) In the hydrogenation of vegetable oils.

(iii) As a rocket fuel in the form of liquid `H_2`.

(iv) In the manufacture of synthetic petrol.

(v) In the preparation of many compounds such as `NH_3`, `CH_3OH` Urea etc,

(vi) It is used in the oxy-hydrogen torch for welding if temperature around `2500^oC` is required. It is also used in atomic hydrogen torch
for welding purposes in which temperature of the order of `4000^oC` is required.

(a) `text(Synthesis of ammonia)` :

`N_2 + 3H_2 underset ( 300^oC, 200 atm) oversettext(Cataylst) (->) 2NH_3; undersettext(Promoter : Mo) oversettext(catalyst :Fe)`

(b) `text(Manufacture of methyl alcohol)` :

`CO + 2H_2 underset ( 300^oC, 200 atm) oversettext(Cataylst) (->) CH_3OH` ; catalyst : `ZnO`, `Cr_2O_3`

(c) `text(Manufacture of hydrogen chloride)` : `H_2 + Cl_2 -> 2HCl` (absorbed in water)

(d) `text(Hydrogenation of oils)` : Hydrogen is chiefly used in catalytic hydrogenation of unsaturated (liquid) oils into solid fats for the production of vegetable ghee.

(e) `text(Synthetic petrol)` : Petrol substitutes are obtained by subjecting a paste of powdered coal in crude oil to the action of hydrogen under pressure and in presence of catalyst.

(f) `text(Oxy-hydrogen flame)` : Oxy-hydrogen flame produces a temperature of `2800^oC` while oxy-atomic hydrogen flame produced a temperature of `4000^oC`.

The heat generated is used for melting substances having very high melting points such as quartz, platinum etc, and also for welding purposes.

(g) Hydrogen is used for creating a reducing atmosphere in processes like annealing and deoxidizing.

(h)With Helium, it is used for filling balloons employed for atmospheric study.