Three isotopes of hydrogen exist and all occur naturally.

Name	Symbol	Atomic number	mass number	Nature
Protium	`text()_(1)H^1` or `H`	1	1	Non-radioactive
Deuterium	`text()_(1)H^2` or `D`	1	2	Non-radioactive
Tritium	`text()_(1)H^3` or `T`	1	2	Radioactive

Some methods for preparation of hydrogen gas are as follows :

(i) Laboratory preparation of dihydrogen : `Zn +H_2SO_4 → ZnSO_4+H_2 ↑`

Hydrogen is collected over water by upward displacement as `H_2` is lighter than water.

(iii) By the action of acids on metal : Those metals which are placed above the hydrogen in the electrochemical series or have positive value of standard oxidation potential `[E_(oxid)^0]` liberate hydrogen gas from acids.

e.g. `undersettext(Granulated zinc)(Zn) +H_2SO_4 (dil) → ZnSO_4 (aq) +H_2 (g) ↑`

`Fe + 2HCl (aq) → FeCl_2 +H_2 ↑`

Some amount of `SO_2` is also produced in this reaction because `H_2` reduced `H_2SO_4` to `SO_2`.

(ii) By the electrolysis of water : Dihydrogen of high purity is prepared by the electrolysis of water in the presence of a small amount of acid or base. (During electrolysis dihydrogen is collected at cathode).

e.g. `2H_2O(l) underset(H^+) oversettext(Electrolysis)→ 2H_2(g) +O_2 (g)`


The reactivity of metals decreases as follows

`K > Ca > Na > Mg > AI > Mn > Zn > Cr > Fe > Co > Ni > Pb > H > Cu > Ag > Au`

(iv) By the action of alkali on metals : Metals like `Zn, Mg, Sn` etc. liberates hydrogen on reaction with alkalies.

e.g. `2Al +2NaOH +2H_2O → 2NaAlO_2+3H_2 (g)`

`Sn+2NaOH +H_2O → Na_2SnO_3+2H_2`

`Zn+2NaOH → Na_2ZnO_2+H_2 (g)`

(v) By the action of water on metals : Very active metals like `Na, K` and `Ca` react with water at room temperature.

`2Na + undersettext(Cold)(2H_2O) → 2NaOH+H_2 ↑` (so, sodium is kept under kerosene oil)

Less reactive metals like `Zn, Mg, Al` etc. can react with water only upon heating.

`Mg + H_2O oversettext(Heat)→ MgO + H_2 ↑`

`2Al + 6H_2O → 2Al (OH)_3 + 3H_2 ↑`

(vi) Bosch process : Manufacturing method : Dihydrogen is prepared by passing steam over red hot coke at 1270 K.

`underset( Coke at 400-600^0 C) C+undersettext(Superheated steam) (H_2O) → undersettext(water gas or syngas)(CO+H_2)`

`undersettext(water gas) (H_2+CO) + undersettext(steam)(H_2O) underset(Fe_2O_3+Cr_2O_3) overset(773 K) → CO_2+2H_2`
`CO_2` is removed either by dissolving in water under pressure of 25-30 atm or by scrubbing the mixture with sodium arsenite.

(vii) Lane's process : Hydrogen is prepared by passing the alternate currents of steam and water gas over red hot iron. This method has two stages :

(a) Oxidation stage : Super heated steam is passed over iron filings heated about 1025-1075 K.

`undersettext(Iron filings)(3Fe) +undersettext(steam) (4H_2O) overset(1025 - 1075 K) → undersettext(Magnetic oxide of iron) (Fe_3O_4) +4H_2+161KJ`

(b) Reduction stage : When the whole iron has been oxidised then steam is stopped and water gas is passed to reduce `Fe_3O_4`.

`Fe_3O_4+4CO → 3Fe +4CO_2`

`Fe_3O_4+4H_2 → 3Fe +4H_2O`

Some physical properties of hydrogen are given below :

`=>` lt is a colourless, tasteless and odourless gas.

`=>` It is slightly soluble in water because its molecules are non-polar. Hydrogen forms diatomic molecules `H_2` joined by very strong covalent bond with bond enthalpy `435.9 kJmol^(-1)`.

`=>` It is the lightest known substance. Its relative density is 0.0695.

`=>` Its melting point is 13.8 K and boiling point is 20.4 K.

`=>` It is highly combustible.

`=>` It can be liquefied at very low temperature and high pressure.

`=>` Metals like Ni, Pt, Pd, Fe, Au etc. can adsorb or occlude large volumes of `H_2` gas at different temperatures (Pd can occlude 1000 times of its own volume).

Some chemical properties of hydrogen are given below :

(i) Dihydrogen is quite stable due to its high bond dissociation energy thus it is not very reactive.

`H_2 overset(2000 K)→ H+ H`

(ii) Dihydrogen is neutral to litmus paper.

(iii) Action with metals : It combines with metals like `Na, K, Ca` etc., to give corresponding interstitial hydrides which are ionic in nature.

`2Na+H_2 overset(Delta)→ 2NaH`

`Ca+H_2 overset(Delta)→ CaH_2`

(iv) Reaction with metal oxides : Dihydrogen acts as a strong reducing agent and thus it reduces metal oxides to metal.

`CaO+H_2 overset(Delta)→ Cu+H_2O`

`ZnO+H_2 overset(Delta)→ Zn+H_2O`

`PbO+H_2 overset(Delta)→ Pb +H_2O`

(v) Reaction with non-metals : The reactivity of halogens towards dihydrogen decreases as `F_2 < Cl_2 < Br_2 > I_2`.

`H_2+F_2 oversettext(Dark)→ 2HF`

`H_2+Cl_2 oversettext(Diffused sunlight)→ 2HCl`

`H_2+Br_2 overset(673 K)→2HBr`

`H_2+I_2 overset(673K)→2HI`

`H_2+S overset(700K)→ H_2S`

(vi) Reaction with unsaturated hydrocarbons : Dihydrogen reacts with unsaturated hydrocarbons to give saturated hydrocarbons.

`undersettext(Ethylene)(H_2C = CH_2+H_2) underset(473 K) overset( Ni or Pt or Pd)→ undersettext(Ethane)(H_3C - CH_3)`

`undersettext(Ethylene) ( HC equiv CH ) +2H_2 underset(473 K) overset(Ni or Pt or Pd) → undersettext(Ethane)(H_3 - CH_3)`

This reaction is used in the hydrogenation of oils.

`undersettext(Liquid) ( text(Vegetable oil) ) + H_2 underset(473 K) overset(Ni) → undersettext(Solid) ( text(Fat))`

The vegetable ghee such as Dalda are prepared by this process.

`=>` It is used in the manufacture of synthetic petrol.

`=>` It is used as a rocket fuel in the form of liquid hydrogen which does not cause any pollution and produces greater energy per unit mass of fuel.

`=>` Over two-thirds of this hydrogen produced is used to prepare ammonia (`NH_3`) by the Haber process.

`=>` It is used in the hydrogenation of vegetable oils.

`=>` It is used as a reducing agent in the laboratory and industry.

`=>` It is used in the preparation of compounds like ammonia (Haber's process), water gas, and fertiliser such as urea, ammonium sulphate etc.

`=>` It is used in the preparation of many organic compounds such as methanol.

`CO(g) +2H_2 (g) undersettext(Catalyst) overset(Cu//ZnO-Cr_2O_3)→ CH_3 OH (l)`

`=>` A molecule of dihydrogen contains two atoms, the nuclei of both the atom in each molecule are spining.

`=>` Molecules of hydrogen in which spins of both the nuclei are in same directions are called ortho hydrogen.

`=>` Molecules in which both the nuclei spin in the opposite directions are called para hydrogen.

`=>` Para hydrogen has lower energy and at `0^0K` the gas contains 100 % para hydrogen.

`=>` At room temperature, the ratio of ortho to para hydrogen is 3 : 1.

`=>` Ortho and para hydrogen, both have similar chemical properties but their properties like boiling points, heating conductivity and specific heat etc. are different.