Hydrogen and Compounds


  • Hydrogen is the first element in the periodic table and the lightest of all elements.
  • It is a colorless, odorless, and tasteless gas.
  • It is highly reactive and can form various compounds with other elements.
  • It is the most abundant element in the universe, but on Earth, it is mostly found in combination with other elements such as oxygen, carbon, and nitrogen.




  • Hydrogen is a chemical element with the symbol H and atomic number 1. It is the lightest and the first element in the periodic table. However, the position of hydrogen in the periodic table is somewhat controversial, and it is often placed separately from other elements in a group of its own.
  • Hydrogen has some similarities with the alkali metals, such as its ability to form a cation with a single positive charge (H+), and with the halogens, such as its ability to form a diatomic molecule (H2) like chlorine and fluorine. However, unlike the alkali metals, hydrogen has no metallic properties, and unlike the halogens, it can lose or gain electrons to form cations or anions, depending on the conditions.
  •  Moreover, hydrogen has both metallic and non-metallic characteristics, and it can form covalent bonds with non-metals, as well as ionic bonds with metals. Therefore the Position for hydrogen is not fixed in the periodic table.


Occurrence of dihydrogen

Dihydrogen, also known as hydrogen gas, is the most abundant chemical substance in the universe, accounting for about 75% of its elemental mass. However, in its molecular form, H2, it is relatively rare on Earth, making up only about 0.0001% of the Earth's atmosphere by volume. It is found in small quantities in the Earth's crust, but is mostly bound up in compounds such as water, hydrocarbons, and biomass. It is also an important component of many stars and gas giants, and is present in significant amounts in some planets, moons, and comets.


Isotopes of Hydrogen


There are three known isotopes of hydrogen:

1.     Protium (symbol H or 1H): This is the most common isotope of hydrogen, accounting for more than 99% of all hydrogen in the universe. It consists of one proton and one electron, with no neutrons in the nucleus.

2.     Deuterium (symbol D or 2H): This is a rare, stable isotope of hydrogen that makes up less than 1% of all hydrogen. It has one proton and one neutron in the nucleus, along with one electron.

3.     Tritium (symbol T or 3H): This is a radioactive isotope of hydrogen that is extremely rare in nature. It has one proton and two neutrons in the nucleus, along with one electron. Tritium is produced in nuclear reactions and has a half-life of about 12.3 years, meaning that half of any given amount of tritium will decay in that time.

The different isotopes of hydrogen have slightly different properties due to differences in their atomic structure. For example, deuterium has a slightly greater mass than protium and can form stronger chemical bonds, while tritium is radioactive and can be used in nuclear reactions.



Ortho and Para Hydrogen :


Hydrogen molecule contains two hydrogen atoms, each atom has one proton in the nucleus with an electron. Like electron, proton is also spinning about its axis.

If two protons in the hydrogen molecule have spins in the same direction then the form is termed as ortho hydrogen and if the protons spins are in opposite direction, the form is known as para hydrogen.

Ortho hydrogen

Para hydrogen

Parallel nuclear spin

Antiparallel nuclear spin

Nuclear spin = ½ + ½ =1

Nuclear spin = ½ - ½ = 0

More stable at room temperature

More stable at low temperature



Laboratory Preparation of dihydrogen

One of the most common laboratory methods of preparing hydrogen gas is through the reaction of a metal with an acid. The general procedure is as follows:


  • A metal such as zinc or magnesium
  • Dilute hydrochloric acid or sulfuric acid
  • A reaction vessel, such as a test tube or flask
  • Delivery tube
  • Water
  • Gas collection apparatus, such as a gas syringe or inverted graduated cylinder


1.     Add small pieces of the metal to the reaction vessel.

2.     Add the dilute acid to the vessel, enough to cover the metal.

3.     Quickly fit the delivery tube to the vessel, making sure that the other end is submerged in a container of water.

4.     Allow the reaction to proceed until gas production ceases or slows significantly.

5.     Collect the hydrogen gas by displacement of water in the container.

6.     Record the volume and mass of the gas collected.

The chemical reaction that occurs is as follows:

Zn (s) + 2HCl (aq) -> ZnCl2 (aq) + H2 (g)

In this reaction, the metal displaces the hydrogen from the acid, producing hydrogen gas and a metal salt. The hydrogen gas is then collected and measured using a gas collection apparatus. This method can also be used with other metals and acids to produce hydrogen gas.


Electrolysis of water: Hydrogen gas can be produced by the electrolysis of water using a Hoffman apparatus. This involves passing an electric current through water, which decomposes into its constituent gases, hydrogen and oxygen.

Reaction of metal hydrides with water: Metal hydrides such as sodium borohydride or lithium aluminum hydride can react with water to produce hydrogen gas. This method is often used for producing small amounts of high-purity hydrogen gas.

Reaction of metal with steam: Certain metals such as iron or zinc can react with steam to produce hydrogen gas and metal oxides. This method is less commonly used than the other methods listed above, but can be effective for producing larger quantities of hydrogen gas.


Commercial Production of dihydrogen

The commonly used processes are outlined below:

(i) Electrolysis of acidified water using platinum electrodes gives hydrogen.


(ii) High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.

 (iii) It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution. During electrolysis, the reactions that take place are:

at anode: 2Cl–(aq) → Cl2(g) + 2e–

at cathode: 2H2O (l) + 2e–→ H2(g) + 2OH–(aq)

The overall reaction is

2Na+ (aq) + 2Cl–(aq) + 2H2O(l)

Cl2(g) + H2(g) + 2Na+ (aq) + 2OH–(aq)

(iv) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.




The mixture of co and H2 is called water gas. As this mixture of CO and H2 is used for the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas or ‘syngas’Nowadays ‘syngas’ is produced from sewage, saw-dust, scrap wood, newspapers etc. The process of producing ‘syngas’ from coal is called ‘coal gasification’.


The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.


This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.

Presently ~77% of the industrial dihydrogen is produced from petro-chemicals, 18% from coal, 4% from electrolysis of aqueous solutions and 1% from other sources.




Physical Properties of dihydrogen


Physical properties : 


1.     Colorless and odorless: Hydrogen is a colorless and odorless gas at room temperature and standard pressure.

2.     Low density: Hydrogen gas has a very low density, about 1/14th that of air. This makes it lighter than air and it will rise in the atmosphere.

3.     Low boiling and melting points: Hydrogen has a low boiling point of -252.87°C and a low melting point of -259.14°C. This makes it one of the few substances that exist as a gas at room temperature.

4.     Highly flammable: Hydrogen gas is highly flammable and can ignite in air when it is mixed with a sufficient amount of oxygen. This makes it useful as a fuel for combustion engines and as a rocket propellant.

5.     Poor conductor of heat and electricity: Hydrogen gas is a poor conductor of heat and electricity.

6.     Highly compressible: Hydrogen gas is highly compressible and can be stored in high-pressure tanks for use as a fuel.

7.     Forms diatomic molecules: Hydrogen gas exists as diatomic molecules (H2), meaning that two hydrogen atoms are bonded together to form a stable molecule.

8.     Low solubility in water: Hydrogen gas has a low solubility in water, which means that it is not easily dissolved in water. 


Chemical Properties of dihydrogen

Reaction of hydrogen with halogens

 It reacts with halogens, X2 to give hydrogen halides, HX, 


While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.


Reaction of hydrogen with dioxygen

It reacts with dioxygen to form water. The reaction is highly exothermic.

2H2(g) + O2 (g)  2H2O(l);

= –285.9 kJ mol–1


Reaction of hydrogen with dinitrogen

With dinitrogen it forms ammonia.


This is the method for the manufacture of ammonia by the Haber process.


Reactions of hydrogen with metals

With many metals it combines at a high temperature to yield the corresponding hydrides (section 9.5)

H2(g) +2M(g)  2MH(s);

where M is an alkali metal


Reactions of hydrogen  with metal ions and metal oxides

It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals.

Reactions of hydrogen with organic compounds

It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example :

(i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee)

(ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.




Uses of Dihydrogen


 The largest single use of dihydrogen is in the synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers.

 Dihydrogen is used in the manufacture of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like soyabean, cotton seeds etc.

 It is used in the manufacture of bulk organic chemicals, particularly methanol.


 It is widely used for the manufacture of metal hydrides (Section 9.5)

 It is used for the preparation of hydrogen chloride, a highly useful chemical.

 In metallurgical processes, it is used to reduce heavy metal oxides to metals.

 Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes. Atomic hydrogen atoms (produced by dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the temperature of 4000 K.

 It is used as a rocket fuel in space research.

 Dihydrogen is used in fuel cells for generating electrical energy. It has many advantages over the conventional fossil fuels and electric power. It does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline and other fuels.



Hydrides are chemical compounds that contain hydrogen and other elements. Depending on the type of hydride, the hydrogen atom can have a negative or positive charge, or be covalently bonded to another atom. There are three types of hydrides:


Ionic or Saline Hydrides

Ionic or Saline Hydrides: Ionic hydrides are formed by the reaction of hydrogen with highly electropositive metals, such as Group 1 and Group 2 metals. These hydrides have a high melting and boiling point and are often insoluble in water. They are also highly reactive and can react with water to produce hydrogen gas.


Molecular Hydride

Covalent or Molecular Hydrides: Covalent hydrides are formed by the sharing of electrons between hydrogen and another nonmetallic element. These hydrides can be classified as polar or nonpolar depending on the electronegativity difference between the two atoms. Examples of covalent hydrides include methane (CH4), ammonia (NH3), and water (H2O).


Molecular hydrides are further classified according to the relative numbers of electrons and bonds in their Lewis structure into : (i) electron-deficient, (ii) electron-precise, and (iii) electron-rich hydrides. 

  • Electron-deficient hydrides have too few electrons for writing their conventional Lewis structure. They are formed by elements of group 13 and act as Lewis acids or electron acceptors.
  • Electron-precise hydrides have the required number of electrons to write their conventional Lewis structures. They are formed by elements of group 14 and have tetrahedral geometry.
  • Electron-rich hydrides have excess electrons present as lone pairs. They are formed by elements of group 15-17 and act as Lewis bases or electron donors. The presence of lone pairs on highly electronegative atoms like N, O and F in hydrides results in hydrogen bond formation between the molecules, leading to their association.


Metallic hydrides

Metallic Hydrides: Metallic hydrides are formed by the reaction of hydrogen with metals, usually in the presence of heat or pressure. These hydrides are often used as hydrogen storage materials, as they can release hydrogen gas when heated. They can also be used as catalysts in certain chemical reactions.