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NCERT Class 11 Chemistry Textbook Chapter 4 With Answer PDF Free Download
Chapter 4: Chemical Bonding and Molecular Structure
Matter is made up of one or different types of elements. Under normal conditions, no other element exists as an independent atom in nature, except noble gases.
However, a group of atoms is found to exist together as one species having characteristic properties. Such a group of atoms is
called a molecule.
Obviously, there must be some force that holds these constituent atoms together in the molecules. The attractive force which holds various constituents (atoms, ions, etc.) together in different
chemical species is called a chemical bond. Since the formation of chemical compounds takes place as a result of the combination of atoms of various elements in different ways, it raises many questions.
Why do atoms combine?
Why are only certain combinations possible? Why do some atoms combine while certain others do not? Why do molecules possess definite shapes?
To answer such questions different theories and concepts have been put forward from time to time.
These are the Kössel-Lewis approach, Valence Shell Electron Pair Repulsion (VSEPR) Theory, Valence Bond (VB) Theory, and Molecular Orbital
(MO) Theory.
The evolution of various theories of valence and the interpretation of the nature of chemical bonds have closely been related to the developments in the understanding of the structure of atoms, the electronic configuration of elements, and the periodic table.
Every system tends to be more stable and bonding is nature’s way of lowering the energy of the system to attain stability.
4.1 KÖSSEL-LEWIS Approach to Chemical Bonding
In order to explain the formation of chemical bonds in terms of electrons, a number of attempts were made, but it was only in 1916 when Kössel and Lewis succeeded independently in giving a satisfactory explanation.
They were the first to provide some logical explanation of valence which was based on the inertness of noble gases.
Lewis pictured the atom in terms of a positively charged ‘Kernel’ (the nucleus plus the inner electrons) and the outer shell that could accommodate a maximum of eight electrons.
He further assumed that these eight electrons occupy the corners of a cube that surround the ‘Kernel’.
Thus the single outer shell electron of sodium would occupy one corner of the cube, while in the case of a noble gas all the eight corners would be occupied.
This octet of electrons represents a particularly stable electronic arrangement. Lewis postulated that atoms achieve the stable octet when they are linked by chemical bonds.
| Author | NCERT |
| Language | English |
| No. of Pages | 36 |
| PDF Size | 12.7 MB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 11 Chemistry Chapter 4 Chemical Bonding and Molecular Structure
Q-1) Explain the formation of a chemical bond
Ans.)
“Chemical bond is an attractive force that bounds the constituents of a chemical species together.”
So many theories are suggested for chemical bond formation such as valence shell electron pair repulsion theory, electronic theory, molecular orbital theory and valence bond theory.
The formation of a chemical bond is credited to the tendency of a system to achieve stability. It was noticed that the inertness of noble gasses is the direct result of their completely filled outermost orbitals.
Consequently, it was proposed that the elements having a deficiency of electrons in outermost shells are unstable.
Thus, atoms combine with one another and finish their separate octets or duplets to achieve the stable configuration of the closest inert gasses.
So, this combination may occur either by sharing electrons. The formed chemical bond is a result of sharing of electrons among atoms known as a covalent bond. Also, a formed ionic bond is a result of sharing of electrons among atoms.
Q-2) How do you express the bond strength in terms of bond order?
Ans.)
The extent of bonding that occurs between two atoms while forming a molecule is represented by bond strength. As the bond strength increases the bond becomes stronger and the bond order increases.
Q-3) Define Bond length.
Ans.)
“Bond length is defined as the equilibrium distance between the nuclei of 2 bonded atoms in a molecule.”
Q-4) Write the significance/applications of the dipole moment.
Ans.)
The following are some of the key significance of the dipole moment:
- The molecule’s shape can be determined. Symmetrical molecules, such as linear, have zero dipole moments, whereas non-symmetrical molecules take on varied shapes, such as bent or angular.
- In order to determine the polarity of molecules. The polarity will be greater if the dipole moment is greater, and vice versa.
- We can say that if a molecule has zero dipole moment, it is non-polar, and if it has some polar character, it is non-polar.
Q-5) Explain with the help of a suitable example polar covalent bond.
Ans.)
When two unique atoms having distinct electronegativities join to form a covalent bond, the bond pair of electrons are not shared equally.
The nucleus of an atom having greater electro-negativity attracts the bond pair. So, the electron distribution gets distorted and an electronegativity atom attracts the electron cloud.
Thus, the electronegative element gets slightly negatively charged and on the other hand, the other atom gets slightly positively charged. As a result of this, two opposite poles are developed in a molecule and this type of bond formed is termed a ‘polar covalent bond’.
For E.g. HCl is having a polar covalent bond. In HCl, Cl- atom is having more electronegativity than H- atom. Thus, the bond pair shifts towards the Cl- atom, and because of that, it acquires a positive charge.
Chemical Bonding and Molecular Structure Textbook With Solutions PDF Free Download