Science Class 11 - Chemical Bonding-And-Molecular-Structure Notes

Chemical Bonding and Molecular Structure

Welcome to the chapter on Chemical Bonding and Molecular Structure for Class 11. In this chapter, you will learn about how atoms combine to form molecules, the different types of chemical bonds, and the shapes of molecules. By the end of this chapter, you will understand the basic concepts of bonding and molecular geometry, which are essential for studying chemistry.

Introduction

Atoms combine to form molecules and compounds. The force that holds atoms together in a molecule is called a chemical bond. Understanding chemical bonding helps explain the properties and behavior of substances.

Key Concepts

• Chemical Bond: The attractive force that holds atoms or ions together in a molecule or compound.
• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight electrons in their outer shell.
• Lewis Structures: Diagrams that show the bonding between atoms and the lone pairs of electrons.

Types of Chemical Bonds

• Ionic Bond: Formed by the transfer of electrons from one atom to another (e.g., NaCl).
• Covalent Bond: Formed by the sharing of electrons between atoms (e.g., H₂O, O₂).
• Coordinate (Dative) Bond: A type of covalent bond where both electrons come from the same atom (e.g., NH₄⁺).
• Metallic Bond: Bonding in metals where electrons are free to move throughout the structure.

Lewis Structures and the Octet Rule

Lewis structures help visualize how atoms share or transfer electrons. Most atoms try to achieve a stable octet configuration.

• Draw dots to represent valence electrons around atomic symbols.
• Show shared pairs (bonds) and lone pairs.

VSEPR Theory and Molecular Shapes

The Valence Shell Electron Pair Repulsion (VSEPR) Theory helps predict the shape of molecules based on the repulsion between electron pairs.

• Linear: 2 bonding pairs (e.g., CO₂)
• Trigonal Planar: 3 bonding pairs (e.g., BF₃)
• Tetrahedral: 4 bonding pairs (e.g., CH₄)
• Trigonal Bipyramidal: 5 bonding pairs (e.g., PCl₅)
• Octahedral: 6 bonding pairs (e.g., SF₆)

Bond Parameters

• Bond Length: The distance between the nuclei of two bonded atoms.
• Bond Angle: The angle between two bonds at an atom.
• Bond Energy: The energy required to break a bond.

Polarity of Bonds and Molecules

When atoms with different electronegativities form a bond, the bond may be polar (e.g., H₂O). Nonpolar bonds occur between atoms with similar electronegativities (e.g., O₂).

Practice Questions

1. Define ionic and covalent bonds with examples.
2. Draw the Lewis structure for CO₂ and H₂O.
3. What is the octet rule? Give an example.
4. Predict the shape of CH₄ using VSEPR theory.
5. Explain why water is a polar molecule.

Challenge Yourself

• Compare the properties of ionic and covalent compounds.
• Explain the formation of NH₄⁺ ion using coordinate bonding.
• Draw the Lewis structure and predict the shape of SF₆.

Did You Know?

• Diamond and graphite are both forms of carbon but have different bonding and structures.
• Water’s bent shape makes it a universal solvent.

Glossary

• Electronegativity: The ability of an atom to attract electrons in a bond.
• Valence Electrons: Electrons in the outermost shell of an atom.
• VSEPR Theory: A model used to predict the shape of molecules.

Answers to Practice Questions

1. Ionic bond: Transfer of electrons (e.g., NaCl). Covalent bond: Sharing of electrons (e.g., H₂O).
2. CO₂: O=C=O; H₂O: H–O–H (with two lone pairs on O).
3. Atoms try to have 8 electrons in their outer shell. Example: NaCl, where Na loses and Cl gains an electron.
4. CH₄ is tetrahedral in shape.
5. Because of the difference in electronegativity and bent shape, water has a positive and a negative end (polar).

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Understanding chemical bonding helps you explain the structure and properties of all substances in chemistry!