NIOS Class 12 Chemistry Chapter 4 Chemical Bonding

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NIOS Class 12 Chemistry Chapter 4 Chemical Bonding

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Chemical Bonding

Chapter: 4

Module – II: Atomic Structure and Chemical Bonding

INTEXT QUESTION 4.1

1. Define electrovalent bond.

Ans: An electrovalent bond is formed when one or more electrons are transferred from one atom to another atom or atom.

2. Show the formation of a nitrogen molecule from two nitrogen atoms in terms of Lewis theory.

Ans: Formation of a Nitrogen Molecule (N₂) Using Lewis Theory:

Step 1: Electronic configuration of nitrogen atom.

(i) Atomic number of nitrogen (N) = 7

(ii) Electronic configuration: 1s² 2s² 2p³

(iii) So, nitrogen has 5 valence electrons (in 2s²2p³)

Step 2: Lewis dot structure of nitrogen atom.

Each nitrogen atom is represented with 5 valence electrons as dots:

N : (1 lone pair and 3 unpaired electrons)

Or simplified as:

: N •• (showing 5 valence electrons around N)

Step 3: Bond formation between two nitrogen atoms:

• Each nitrogen atom needs 3 more electrons to complete its octet.

• So, both atoms share 3 pairs of electrons, forming a triple bond.

Lewis structure of N₂ molecule:

• Each nitrogen shares 3 electrons with the other (triple bond: one sigma and two pi bonds).

• Each nitrogen also retains one lone pair (2 electrons).

Final Lewis Structure:

Explanation:

• The triple covalent bond between the two nitrogen atoms satisfies the octet rule for both atoms.

• This strong triple bond makes N₂ a very stable and inert molecule, consistent with its behavior in nature (78% of air is N₂).

3. What do you understand by a polar covalent bond? Give two examples.

Ans: A polar covalent bond is a type of chemical bond where a pair of electrons is unequally shared between two atoms. This uneven sharing creates a dipole moment, with one atom having a slightly negative charge (δ-) and the other a slightly positive charge (δ+). This occurs when the atoms have different electronegativities.

Examples include:

(i) Water (H2O): Oxygen is more electronegative than hydrogen, so it pulls the shared electrons closer, creating a polar bond.

(ii) Hydrogen chloride (HCl): Chlorine is more electronegative than hydrogen, resulting in a polar bond.

4. What is a coordinate covalent bond ? How is it different from a covalent bond?

Ans: A bond in which both the bonding electrons are contributed by one atom only.

INTEXT QUESTION 4.2

1. What are the basic postulates of VSEPR theory?

Ans: The two postulates of VSEPR theory are:

(i) The electron pairs (both bonding and non-bonding) around the central atom in a molecule arrange themselves in space in such a way that they minimize their mutual repulsion.

(ii) The repulsion of a lone pair of electrons for another lone pair is greater than that between a bond pair and a lone pair which in turn is stronger than that between two bond pairs The order of repulsive force between different possibilities is as under.

lone pair – lone pair > lone pair – bond pair > bond pair – bond pair.

2. Predict the shape of methane (CH₄) on the basis of VSEPR theory.

Ans: In methane the central carbon atom would have four pairs of electrons in its valence shell. According to VSEPR theory these would be placed tetrahedrally around the carbon atom. Hence the methane molecule would have a tetrahedral shape.

3. It is a molecule the difference between the electro-negativity of two atom is 1.7. How much % will be ionic and covalent character?

Ans: 50% ionic and 50% covalent character.