NCERT Solutions for Class 11 Chemistry Notes

CLASS 11 CHEMISTRY NOTES INTRODUCTION Chemical Bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. The explanation of the attractive forces is a complex area that is described by the laws of quantum electrodynamics. In general, strong chemical bonding is associated with the sharing or transfer of electrons between the participating atoms.

The study on the “nature of forces that hold or bind atoms together to form a molecule” is required to gain knowledge of the following • To know about how atoms of same element form different compounds combining with different elements.

• To know why particular shapes are adopted by molecules.

• To understand the specific properties of molecules or ions and the relation between the specific type of bonding in the molecules.

1. LEWIS THEORY The Lewis theory gave the first explanation of a covalent bond in terms of electrons that was generally accepted. If two electrons are shared between two atoms, this constitutes a bond and binds the atoms together. For many light atoms, a stable arrangement is attained when the atom is surrounded by eight electrons.

The octet can be made up from some electrons which are totally owned and some electrons which are ‘shared’. Thus atoms continue to form bonds until they have made up an octet of electrons. This is called the ‘octet rule’. The octet rule explains the observed valences in a large number of cases. There are exceptions to the octet rule; for example, hydrogen is stable with only two electrons.

The conventional Lewis structure representation of a pair of bonded electrons is by means of a ‘dash’ (-) usually called a ‘bond’. Lone pairs or ‘non-bonded’ electrons are represented by ‘dots’. Some structures are represented below: H

..

H : N : H ..

H

(NH3)

or

H

N ..

H .. : O

(CO2)

C

..

O :

(CH3CHO)

H

C

H

H C

..

O :

H

Such representations of organic molecules are not usually problematic. However, ‘hit-and-trial’ is generally the method (obviously not very efficient) used by most students in figuring out the structures of inorganic molecules.

2. LEWIS DOT STRUCTURES The formula of a molecule shows the number of atoms of each element but does not show the bonding arrangement of the atoms. To represent the bonding pattern in a molecule, the electron dot symbols of the elements are arranged such that the shared pairs and unshared pairs (called lone pairs) are shown and the octet rule (or duet for hydrogen) is satisfied. For example, .. ..

..

a molecule of fluorine is shown as : F.. : F .. :or : F .. as

..

H : F.. :

or

H

..

F .. : and a molecule of hydrogen fluoride is shown

..

F.. : .

Arrangement of dot symbols used to represent molecules are called Lewis structures. Lewis structures do not convey any information regarding the shape of the molecule. Usually, the shared

pairs of electrons are represented by lines between atoms and any unshared pairs are shown as dot pairs.

Lewis structures are written by fitting the element dot symbols together to show shared electron pairs and to satisfy the octet rule. For example, (i)

.. . .

In water (H2O), one Hand two O . : complete their duet and octet respectively

.. :O

H

H ..

(ii)

..

.

In ammonia (NH3), three H and one . N. . fit together and satisfy their duet and octet H

N

respectively as

H .. :Cl: ..

(iii)

.

. . . complete their octet as In carbon tetrachloride (CCl4), four : Cl .. and one C .

.. : Cl ..

C

..

Cl .. :

:Cl .. :

For the given molecules, we have adopted hit & trial method to fit the dot symbols together and satisfy the octet rule. But remember that hydrogen form one bond, oxygen forms two bonds, nitrogen three bonds and carbon forms four bonds. For simpler molecules, the hit & trial method works perfectly but for slightly complicated polyatomic species, this may give us more than one possible structure. Thus, a systematic approach is needed to design the Lewis structures of such polyatomic species. But before proceeding further, let us understand the limitation of this approach.

2.1 LIMITATIONS OF LEWIS THEORY OF DRAWING STRUCTURE This method would be applicable to only those molecules/species, which follow octet rule except hydrogen.There are three kinds of molecules/species, which do not follow octet rule. (a) Molecules, which have contraction of octet. Such molecules are electron deficient. For example, BH3, BF3, BCl3, AlCl3, GaCl3 etc. (b) Molecules, which have expansion of octet. Such species have more than eight electrons in their outermost shell. This is possible in those molecules, which have vacant d-orbitals, thus they can expand their octet. For example, PCl5, SF6 etc. (c) Molecules containing odd number of electrons (in total) cannot satisfy octet rule. Such species are called odd electron species and are paramagnetic in nature due to presence of unpaired electron.

For example, NO, NO2 and ClO2.

H