Hydrogen Chapter 9 Class 11 Chemistry NCERT Textbook PDF

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NCERT Class 11 Chemistry Textbook Chapter 9 With Answer PDF Free Download


Chapter 9: Hydrogen

Hydrogen has the simplest atomic structure among all the elements around us in Nature. In atomic form, it consists of only one proton and one electron.

However, in the elemental form, it exists as a diatomic (H2) molecule and is called dihydrogen. It forms more compounds than any other element.

Do you know that the global concern related to energy can be overcome to a great extent by the use of hydrogen as a source of energy? In fact, hydrogen is of great industrial importance as you will learn in this unit.

9.1 Position of Hydrogen In The Periodic Table

Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past.

As you know by now that the elements in the periodic table are arranged according to their electronic configurations.

9.2.1 Occurrence

Dihydrogen is the most abundant element in the universe (70% of the total mass of the universe) and is the principal element in the solar atmosphere. The giant planets Jupiter and Saturn consist mostly of hydrogen. However, due to its light nature, it is much less abundant (0.15% by mass) in the earth’s atmosphere.

Of course, in the combined form it constitutes 15.4% of the earth’s crust and the oceans. In the combined form besides in water, it occurs in plant and animal tissues, carbohydrates, proteins, and hydrides including hydrocarbons and many other compounds.

9.4.1 Physical Properties

Dihydrogen is a colorless, odorless, tasteless, combustible gas. It is lighter than air and insoluble in water. Its other physical properties along with those of deuterium are given in Table 9.1.

9.4.2 Chemical Properties

The chemical behavior of dihydrogen (and for that matter any molecule) is determined, to a
large extent, by bond dissociation enthalpy.

The H–H bond dissociation enthalpy is the highest for a single bond between two atoms of any element.

What inferences would you draw from this fact? It is because of this factor that the dissociation of dihydrogen into its atoms is only ~0.081% around 2000K which increases to 95.5% at 5000K.

Also, it is relatively inert at room temperature due to the high H–H bond enthalpy. Thus, the atomic hydrogen is produced at a high temperature in an electric arc or under ultraviolet radiation.

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NCERT Solutions Class 11 Chemistry Chapter 9 Hydrogen

Q 9.1

Justify the position of hydrogen in the periodic table on the basis of its electronic configuration.


The 1st element in the periodic table is hydrogen. Hydrogen exhibits dual behavior because it has only 1 electron on its one ‘S’ shell.(i.e.,) hydrogen resembles both halogens and alkali metals.

Electronic configuration of hydrogen = [1s1]

Hydrogens resemblance with alkali metals:

Hydrogen has 1 valence electron on its valency shell-like alkali metals.

[He] 2s – Li

1s1 – H

[Ne] 3s1 – Na

Therefore, to form a uni positive ion, it can lose one of its electrons.

To form halides, oxides, and sulfides, it combines with electro –elements which are the same as alkali metals.

Hydrogens’ resemblance with halogens:

Only 1 electron is required to complete their respective octets for both the halogen and hydrogen.

H : 1s 1

F : 1s 2 2s 2 2p 5

Cl : 1s 2 2s 2 2p 6 3s 2 3p 5

It forms several covalent compounds and diatomic molecules like halogens. Even though hydrogen has certain similarities among both halogen and alkali metals, it differs from them. Hydrogen won’t possess metallic characteristics; it possesses higher ionization enthalpy and reacts less than halogens.

Due to these reasons, hydrogen cant is replaced with alkali metal of the 1st group or with the halogens of the 2nd group. Therefore, it is best to place hydrogen separate in the periodic table.

 Q 9.2

Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?


3 isotopes:

(i)  tritium   ^{3}_{1}H13​H or T

(ii)  protium ^{3}_{1}H13​H

(iii) deuterium ^{2}_{1}H12​H or D

Mass Ratio:

Tritium : Protium : deuterium = 1 : 2 : 3

Q 9.3

Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?


The ionization enthalpy of a hydrogen atom is higher. Therefore, it is harder to remove its electron. This results in its tendency to exist in the low monoatomic form. Instead of that, a covalent bond is formed by hydrogen with another hydrogen atom and exists as a diatomic molecule.

Q 9.7

Discuss the consequences of high enthalpy of H–H bond in terms of chemical reactivity of dihydrogen

Ans :

The ionization enthalpy of the H–H bond is higher (1312 kJ mol–1 ) which shows that hydrogen has a low tendency to form H+ ions. Its ionization enthalpy value is comparable to that of halogens. Hence, it forms

→ a large number of covalent bonds

→ diatomic molecules (H2)

→ hydrides with element

Hydrogen does not possess metallic characteristics (luster, ductility, etc.) like metals because ionization enthalpy is very high.

Q 9.8 

What do you understand by (i) electron-rich (ii) electron-precise, and (iii) electron-deficient? – compounds of hydrogen, Provide justification with suitable examples.


Molecular hydride is classified on the basis of the presence of the bonds and the total number of electrons in their Lewis structures as:

  1. Electron-deficient hydrides
  2. Electron-precise hydrides
  3. Electron-rich hydrides

1. An electron-deficient hydride has very less electrons, less than that required for representing its conventional Lewis structure.

 E.g BH3, AlH3, etc. 

They exist in diametric forms such as B2H6, and Al2H6 to make up for their deficiency.

In B2 H6, there are 6 bonds in all, out of which only 4 bonds are regular 2 centered-2 electron bonds. The remaining 2 bonds are 3 centered-2 electron bonds i.e., 2 electrons are shared by 3 atoms.

2. An electron-precise hydride has a sufficient number of electrons to be represented to form a covalent bond. 

 E.g. CH4, SiH4, etc.

In this compound 4, regular bonds are formed where 2 electrons are shared by 2 atoms. 

3. An electron-rich hydride compound contains excess valence electrons to form covalent bonds.

 E.g. NH3, PH3

There are 3 regular bonds in all with a lone pair of electrons on the nitrogen atom.

Hydrogen Textbook With Solutions PDF Free Download

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