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Symbol : H

Atomic Number : 1

Atomic weight : 1.008

Electronic configuration : 1s1

Discovered by : Henry Cavendish

Henry Cavendish discovered hydrogen by passing steam through a red hot gun barrel (mostly iron)

  • He called it ‘inflammable air’. It’s name hydrogen was proposed by Lavoiser (Father of modern chem) as it produces water during burning with oxygen( Greek, hydro= water, gen= producer)
  • It is the most abundant and lightest element in the universe { and tenth most abundant element in the earth’s crust}

Position of hydrogen in the periodic table


position of hydrogen in periodic table

It is the first element in the periodic table. It resembles the alkali metals (group IA) as it can be oxidized to H+ ion. On the other hand, it resembles the halogens (group VIIA) as it forms the hydride ion (H). Hence ,the position of hydrogen in the periodic table is controversial or anomalous. Hence, it has been given a separate and distinct place unassociated with any particular group as the first element on left side of the periodic table on the top of the alkali metals (gr.IA).


Different forms of hydrogen


  1. Molecular hydrogen
  2. Atomic hydrogen
  3. Nascent hydrogen
  4. {Occluded hydrogen}
1. Molecular hydrogen 

Ordinary hydrogen gas which consists of diatomic molecules in which two H atoms are combined together by a covalent bond is known as molecular hydrogen. Due to high H-H bond energy ( 104 KCalmol-1), molecules of hydrogen is very stable. It does not reacts under ordinary conditions.


2. Nascent hydrogen 

(nascent=newly born) : The atomic hydrogen which is produced during the chemical reaction is called nascent hydrogen.

It is more reactive than ordinary hydrogen molecule. Thus nascent hydrogen is powerful reducing agent than ordinary hydrogen. Following reactions support this statement:

i. Reduction of potassium permanganate:

When molecular hydrogen gas is passed through acidified KMnO4, the pink colour of KMnO4 does not change.

But when a piece of zinc is treated with acidified KMnO4, it’s pink colour is discharged. This is because nascent hydrogen produced from the reaction between zinc and H2SO4 reduces KMnO4 into MnSO4 salt.

Reduction of potassium permanganate by nascent hydrogen

ii.Reduction of potassium dichromate:

When molecular hydrogen gas is passed through acidified K2Cr2O7, the orange colour of K2Cr2O7 does not change.

But when a piece of zinc is treated with acidified K2Cr2O7, it’s orange colour changes to light green. This is because nascent hydrogen produced from the reaction between zinc and H2SO4 reduces K2Cr2O7 into Cr2(SO4)3 salt.

Reduction of potassium dichromate by nascent hydrogen

iii.Reduction of ferric chloride:

When molecular hydrogen gas is passed through ferric chloride, the yellowish brown(orange) colour of ferric chloride does not change.

But when a piece of zinc is treated with acidified FeCl3, its colour changes to pale green.

Reduction of ferric chloride by nascent hydrogen

Uses : It is used as reducing agent.


3. Atomic hydrogen 

It is hydrogen atom formed by dissociation of molecular hydrogen at about 1000 to 2000oC at low pressure. {This can be done by passing hydrogen gas through an electric arcstruck between two tungsten electrodes}

atomic hydrogen

It has very short half life time of 0.3 second. Thus it is unstable.

It is more powerful reducing agent than molecular hydrogen and nascent hydrogen. It can reduce oxides and chlorides of metals like copper, silver, mercury to metallic form.

reduction by atomic hydrogen

Uses of atomic hydrogen:

i. It is used as reducing agent.

ii. It is used as atomic hydrogen torch for welding metals of very high melting point.


Differences between nascent and atomic hydrogen :

Nascent hydrogen Atomic hydrogen
1. Nascent hydrogen is formed in the reaction mixture at room temperature. 1. Atomic hydrogen is produced at very high temperature of about 2000oC by dissociating molecular hydrogen.
2. Nascent hydrogen is formed in situ but not been isolated yet. 2. Atomic hydrogen can be isolated and has a half life of 0.3 seconds.
3. It is weaker reducing agent than atomic hydrogen . It can not reduce certain metallic salts like AgCl, CuO, etc. 3. It is stronger reducing agent than nascent hydrogen. It can reduce such metallic salts. Eg.
4. Occluded hydrogen :

Adsorption of any gas on metal surface is known as occlusion. When molecular hydrogen is adsorbed on a metal surface (such as Fe, Au, Pt and Pd) , then this hydrogen is known as occluded hydrogen. In the powdered state, Pd can occlude approximately 1000 times its own volume of hydrogen. The occluded gas interacts with the metal to form a solid solution; or sometimes a part of the gas combines with the metal to yield other chemical compounds, for example, hydrides.


Ortho and Para hydrogen


Hydrogen molecule in which two hydrogen atoms have same nuclear spin (parallel nucleus spin) is called ortho hydrogen and hydrogen molecule in which two hydrogen atoms have opposite nuclear spin( anti-parallel nucleus spin) is called para hydrogen.

ortho and para hydrogen

Therefore, in ortho hydrogen two nuclei spin in the same direction and resultant nuclear spin is high but in para hydrogen, two nuclei spin in opposite direction and the resultant nuclear spin is zero. Hence, para hydrogen has lower internal energy than the ortho hydrogen.

  • Due to difference in internal energy, the two forms of hydrogen have different physical properties like boiling point, specific heat, thermal conductivity, etc.
  • Since, para hydrogen has lower energy, at absolute zero, there is 100% para hydrogen and with increase in temp., proportion of para form decreases. At room temperature, hydrogen gas is mixture of 25% of para and 75% of ortho hydrogen.
Differences between ortho and para hydrogen :

Ortho hydrogen Para hydrogen
  • Hydrogen molecule in which two hydrogen atoms have same nuclear spin is called ortho hydrogen.
  • Hydrogen molecule in which two hydrogen atoms have opposite nuclear spin is called para hydrogen.
  • It is more stable at room temperature.
  • It is very stable at very low temperature(273K)
  • 75% of total hydrogens are ortho at room temperature.
  • 25% of total hydrogen are para at room temperature.
  • It’s % can be increased by increasing temperature.
  • It’s % can be increased by decreasing temperature.

Isotopes of Hydrogen


Isotopes are the atoms of the same element having same atomic number but different mass number. There are three isotopes of hydrogen:

  1. Protium or ordinary hydrogen
  2. Deuterium or heavy hydrogen
  3. Tritium or radioactive hydrogen.

isotopes of hydrogen

Name Protium Deuterium Tritium
Symbol 1H or H 2H or D 3H or T
No. of protons(P) 1 1 1
No. of neutrons(n) 0 1 2
No. of electrons(e) 1 1 1
Atomic no.(Z) 1 1 1
Mass no.(A) 1.007 2.0014 3.017

Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium.

  • Isotopes have different physical properties since they differ in their mass number.
  • They have same chemical properties since their electronic configuration is same. However, they differ in the rate of chemical reaction. For example, D2 reacts with Cl2 about 13 times slower than H2 does. The different in rate of reaction due to difference in mass of the atoms of the same element is called isotope effect.

Uses of protium :

  • It is used in filling {toy} balloons.
  • It is used in synthesis of ammonia by Haber’s process.
  • It is used as rocket fuel.
  • It is used for the hydrogenation of vegetable oil to form vanaspati ghee. 

Uses of deuterium :

  • It is used in nuclear fusion reaction to liberate large amount of heat which is used in hydrogen bomb.

use of deuterium

  • It { and its compounds } is used as tracer to determine the mechanism of reactions.
  • It is used to prepare heavy water.

Uses of tritium :

  • It is used in nuclear fusion reaction to liberate large amount of heat which is used in hydrogen bomb.

use of tritium

  • It is used as radioactive tracer { in absorption of water by plant roots.}
  • It is used as radioactive substance in cancer therapy.

Questions and their answers :


1. Which of the following is most reactive form of hydrogen?

a. Nascent hydrogen    c. Ortho hydrogen

b. Atomic hydrogen     d. Para hydrogen

2. Water gas is a mixture of :

a. H2O and CO2    c. H2O and CO

b. H2 and SO2       d. H2 and CO.

3. Which of the following is known as heavy hydrogen?

a. Protium       c. tritium

b. Deuterium  d. para hydrogen

4. Which of the following is known as radioactive hydrogen?

a. Protium        c. tritium

b. Deuterium   d. para hydrogen

5. At room temperature, ordinary hydrogen is the mixture of ortho and para hydrogen in the ratio of :

a. 1:3     c. 2:3

b. 3:1     d. 1:2

6. When a piece of zinc is added to acidified potassium dichromate solution, the color changes to:

a. Yellow    c. sky blue

b. Pink       d. light green

7. Least abundant isotope of hydrogen is:

a. Protium      c. tritium

b. Deuterium d. Heavy hydrogen

8. The adsorption of hydrogen by metals is called:

a. Dehydrogenation   c. Occlusion

b. Hydrogenation       d. Absorption

Answers :

1. b     2. d

3. b     4. c

5. a     6. d

7. c     8. c

Note : The adsorption of hydrogen by metals (such as Fe,Au,Pt and Pd) is called occlusion. In the powdered state, Pd can occlude approximately 1000 times its own volume of hydrogen. 

Q) Give a reaction which shows that water contains hydrogen.

→ When steam is passed through red hot iron, hydrogen gas is obtained.

Since, iron does not contain hydrogen, it must come from water. This reaction shows that water contains hydrogen.

Q) When seam is passed through red hot coke, a mixture of two gases is obtained. Name the gases and write the reaction involved.

→ When steam is passed through red hot coke, a mixture of CO and H2 gas is obtained.

These gases CO and H2 together are called water gas.


References