NEET 2022 | Class 12

NEET Chapter: Hydrogen

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Introduction to Hydrogen

Introduction to Hydrogen

Hydrogen is the most abundant element in the universe and the third most abundant element on Earth's surface. It's also the most basic element, with only one proton and one electron in its orbit around the nucleus. This element exists as a diatomic molecule, H2, in its elemental form.

The most pressing global issue with renewable energy needs a careful investigation of hydrogen as a distinct notion from other elements. The use of hydrogen as a source of energy can help to relieve this concern to a greater extent.

It also resembles halogens and alkali metals in terms of characteristics. Because of its unique properties, it has its own spot in the periodic table. It is also impossible to find it in its native state in the Earth's atmosphere.

Important Topics for Hydrogen

  • Hydrogen

  • Properties of Hydrogen

Important Concepts for Hydrogen

The Element - Hydrogen

  • Hydrogen is the first element in the periodic table.

  • Hydrogen is placed in no specific category because of its property of supplying electrons (when H+ is generated) and withdrawing electrons (when H- is formed).

  • Hydrogen is included in Group I (Alkali metals) as follows:

  • It possesses one electron in its (Outer) shell - 1𝜎1, just like other alkali metals with a (inert gas) 𝜈𝜎1 configuration.

  • Monovalent H+ ions, such as Li+ and Na+, are produced.

  • It also has a valency of one.

  • Its oxide (H2O) is stable in the same way that Na2O is.

  • Similar to Na and Li, it is a good reducing agent (in both the atomic and molecular states).

  • In the following ways, hydrogen resembles halogens (Group VII A):

  • It's also Xi2, which means it's diatomic.

  • It also produces anion H-, which is similar to F- and Cl-, by gaining one electron.

  • As XH4, H has a stable inert gas (He) configuration, while X2H6 contains halogens and other elements.

  • H is one electron short of duplet, like F, Cl, which is also one electron short of octet (stable configuration).

  • H has an ionisation enthalpy (IE) that is analogous to halogens.

  • H has a very high level of alkali metals in comparison (IE). Furthermore, as compared to the size of an alkali metal ion, H+ is quite small.

  • Because of its lower electron affinity, H forms stable hydrides only with strongly electropositive metals.

  • It's difficult to place hydrogen in a specific spot in the periodic table because of its unique behaviour.

  • As a result, it's customary to classify it in both groups I (together with alkali metals) and group VII (along with heavy metals) (Along with halogens).

Hydrogen’s Physical Properties

  • It is a colourless, odourless, and tasteless gas.

  • To some extent, it is water-soluble.

  • It has high flammability.

  • Atomic radius (pm): 37

  • Ionic radius of H- ion (pm): 210

  • Ionisation energy: 1312 kJ/mol.

  • Electron affinity: 72.8 kJ/mol.

  • Electronegativity: 2.1

Hydrogen's Chemical Properties

  • Only at temperatures above 2000 K dihydrogen split into hydrogen atoms, making it a reasonably stable gas,
    H2 → H + H.

  • It has a very high bond dissociation energy of H, ΔH = 435.9 kJ/mol.

  • Due to its high bond dissociation energy, it is not very reactive. However, it reacts with a wide range of elements and compounds.

  • Dihydrogen forms interstitial hydrides with transition metals (d-block elements) like Pd, Ni, and Pt, in which little hydrogen molecules fill the interstitial spaces in the crystal lattices.

  • When H2 combines with unsaturated hydrocarbons like ethylene and acetylene, saturated hydrocarbons are produced.

Hydrogen's Applications

  • As a reducing agent,

  • Vegetable oils are hydrogenated in the process of hydrogenation using edible plants,

  • As a rocket fuel, in the form of liquid H2,

  • Synthetic gasoline is made in a factory and,

  • This method is used to make a variety of chemicals.

Isotopes of Hydrogen

  • The other three isotopes of hydrogen are: (i)  Protium, (ii) Deuterium, (iii) Tritium.

  • Representation is given as: 

  • Protium: 11H or H,

  • Deuterium: 21H or D,

  • Tritium: 31H or T. 

  • Tritium is the sole radioactive isotope among the three; the other two are non-radioactive.

  • Isotopes have chemical properties that are similar in general, yet they differ quantitatively. For example, the atomic number of Deuterium is 1, the same as hydrogen but the valency is 2 and similarly, the atomic number of Tritium is also 1 but the valency stands at 3. 

Different Forms of Hydrogen

  • Atomic hydrogen is created through the dissociation of hydrogen molecules.

  • Atomic hydrogen is incredibly reactive and barely lasts a fraction of a second before exploding.

  • At atmospheric pressure, dihydrogen gas is passed through an electric arc generated between two tungsten rods.

  • Nascent hydrogen is the hydrogen gas created in the reaction mixture when it comes into contact with the substance to be reacted with. "Newly born hydrogen" is another name for it. Its reactivity is stronger than that of normal hydrogen. Nascent hydrogen was also a term used previously to describe dissolving metal reactions but now it is understood that such reactions take place at the metal surface and hence this postulate is discredited. 

  • A hydrogen molecule is made up of two atoms. In each hydrogen molecule, the nuclei of both atoms are spinning.

  • Ortho hydrogen is a hydrogen molecule in which both nuclei's spins point in the same direction.

  • Para-hydrogen is a form of the hydrogen molecule in which the nuclei's spins are in opposite directions.

  • At zero degrees celsius, hydrogen is primarily para-hydrogen, which is more stable.

  • At the temperature of air liquefaction, the ratio of ortho and para-hydrogen is 1:1.

  • At room temperature, the ratio of ortho to para-hydrogen is 3:1.

  • Even at extremely high temperatures, the ratio of ortho to para-hydrogen can never surpass 3:1.

Solved Examples from the Chapter: Hydrogen

Question 1: Because of this property hydrogen is quite similar to halogens.
(a) a powerful reducing agent
(b) it’s diatomic gas
(c) it's a colourless gas.

(d) it has a 0 V reduction potential.


  • When comparing Hydrogen and Halogens, the following are the properties that come up,

  • It is diatomic in nature.

  • It produces an H- ion just like any halogen by gaining one electron and hence it is a strong oxidising agent.

  • It has similar ionisation enthalpy as that of halogens. 

  • From the given options it is clear that the similarity between hydrogen is a diatomic gas. 

  • As a result, the correct answer is (b).

Key points to remember: It is important to know about the properties of both hydrogen and halogen. In comparison, such questions can be answered. 

Question 2: Hydrogen is unable to reduce.
(a) Hot CuO
(b) Fe2O3
(c) Hot SnO2 

(d) Hot Al2O3


  • Of the given options, Hydrogen can easily reduce, Hot CuO, Fe2O3, and Hot SnO2

  • The only element that hydrogen won't be able to reduce is Hot Al2O3.

  • This is due to the fact that Al has a higher affinity for oxygen than hydrogen.

  • As a result, (d) Hot Al2O3 is the correct answer. 

Key points to remember: Hydrogen definitely has a reducing property. But in order to reduce an element, the reduction potential should be more than the enthalpy of the element.

Solved Questions from the Previous Years’ Question Papers

Question 1: Identify the incorrect statement regarding heavy water.

(a) It reacts with SO3 to form deuterated sulphuric acid (D2SO4).

(b) It is used as a coolant in nuclear reactors.

(c) It reacts with CaC2 to produce C2D2 and Ca(OD)2.

(d) It reacts with Al4C3 to produce CD4 and Al(OD)3.


  • In nuclear reactors, heavy water is employed as a moderator to control the speed of neutrons. 

  • It is used to keep things cool.

  • As a result, option (b) is the proper response.

Question 2: Calculate the total number of neutrons in three hydrogen isotopes.

(a) One

(b) Two

(c) Three

(d) Four


  • The neutron number in three hydrogen isotopes = 0 + 1 + 2 = 3

  • As a result, option (3) is the correct answer.

Question 3: Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases does H2O2 act as a reducing agent in acid medium?

(a) MnO4–

(b) SO32-

(c) KI

(d) Cr2O72-


  • In a redox chemical process, a reducing agent is an element or compound that loses or gives an electron to an electron recipient, the oxidising agent.
    H2O2 + MnO4– → Mn+2 + O2

  • As a result, option (a) MnO4– is the correct answer.

Practice Questions

Question 1: The right order of increasing reducing nature is as follows:

(a) NaH > H2O < MgH2 

(b) H2O > MgH2 > NaH

(c) H2O < MgH2 < NaH

(d) H2O = MgH2 < NaH

Answer: (c) H2O < MgH2 < NaH.

Question 2: Under typical circumstances, hydrogen occurs in diatomic rather than monatomic form:

(a) Because of the high enthalpy of ionisation

(b) Because of the low enthalpy of ionisation

(c) Because of the high electron gain enthalpy

(d) Because of the low electron gain enthalpy

Answer: (a) Because of the high enthalpy of ionisation.


As a result, hydrogen has the simplest atomic structure of all the elements discovered in nature. In its atomic state, it is made up of only one proton and one electron. In its elemental form, where it appears as a diatomic (H2) molecule, it is known as hydrogen. It is the element with the greatest number of compounds. 

Are you aware that using hydrogen as a source of energy can assist to address some of the world's energy issues? In reality, as this article has demonstrated, hydrogen is critical in manufacturing. In this article, we also understand what is the natural occurrence of hydrogen in the universe, “hydrogen is the most abundant element in the universe”. It is present in molecular form as well as in many compounds.

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FAQs on NEET Chapter: Hydrogen


1. What will hydrogen be used for?

Hydrogen is now mostly used in oil refining and fertiliser production. In order to make a significant contribution to clean energy transitions, it must also be adopted in sectors where it is currently essentially absent, such as transportation, buildings, and power generation.

2. What is the significance of hydrogen?

Because it powers the sun, which converts hundreds of millions of tonnes of hydrogen into helium every second, hydrogen is essential for human survival. When two hydrogen atoms are bonded to one oxygen atom, water is created. Both of these variables contribute to our planet's habitability.

3. What do we do with hydrogen in our daily lives?

It is used to generate fertilisers, grind metals, and produce ammonia, which is utilised to make synthetic materials such as plastics. When liquid hydrogen is combined with liquid oxygen, it can be used as rocket fuel to produce a strong explosion.