Covalent Bonding & Structures | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive overview of covalent bonding and structures, essential topics in A-level Chemistry. Understanding these concepts is crucial for mastering the subject and excelling in your exams. We will cover what covalent bonds are, how to represent them, the different types of covalent bonding, and the properties of covalent compounds.

Step 1: Understand Covalent Bonds

Covalent bonds are formed through the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms. This typically occurs between two non-metals.

Key Characteristics

• Electrons Shared: In a covalent bond, atoms share electrons to achieve full outer electron shells.
• Electronegativity: Consider the electronegativity of the atoms involved, as it influences bond polarity.

Step 2: Create Dot-Cross Diagrams

Dot-cross diagrams visually represent covalent bonding by showing the sharing of electrons.

How to Draw a Dot-Cross Diagram

1. Identify the atoms involved in the bond.
2. Represent the electrons of one atom as dots and the electrons of the other atom as crosses.
3. Indicate shared pairs with a line connecting them, showing they form a bond.

Example

For two chlorine atoms (Cl):

• Each Cl atom has 7 valence electrons.
• Share one pair to form a bond, resulting in a full outer shell for both.

Step 3: Explore Types of Covalent Bonding

Covalent bonds can be classified into three types:

Single Covalent Bonds

• Involve one shared pair of electrons.
• Example: H2 (two hydrogen atoms share one pair).

Multiple Covalent Bonds

• Involve more than one shared pair of electrons.
• Double Bond: 2 shared pairs (e.g., O2).
• Triple Bond: 3 shared pairs (e.g., N2).

Dative Covalent Bonds

• One atom donates both electrons in the bond.
• Also known as coordinate bonds.
• Example: Ammonium ion (NH4+).

Step 4: Identify Covalent Structures

Covalent compounds can be structured as simple molecular or giant covalent lattices.

Simple Molecular Substances

• Composed of small molecules.
• Held together by strong covalent bonds within molecules and weak intermolecular forces between them.
• Example: Water (H2O), where H2O molecules are held together by weak forces in ice.

Properties of Simple Molecular Substances

• Low Melting and Boiling Points: Due to weak intermolecular forces.
• Solubility: Soluble in non-polar solvents.
• Electrical Conductivity: Non-conductors as there are no free charged particles.

Giant Covalent Substances

• Formed by a network of strong covalent bonds.
• Example: Diamond, graphite, silica (SiO2).

Properties of Giant Covalent Substances

• High Melting and Boiling Points: Strong bonds require significant energy to break.
• Insolubility: Insoluble in both polar and non-polar solvents.
• Electrical Conductivity: Generally non-conductors, with graphite being an exception due to delocalized electrons.

Step 5: Compare Simple Molecular and Giant Covalent Structures

Simple Molecular

• Low melting point.
• Soluble in non-polar solvents.
• Non-conductor of electricity.

Giant Covalent

• High melting point.
• Insoluble.
• Non-conductor of electricity (except graphite).

Conclusion

Understanding covalent bonding and the structures of compounds is foundational in chemistry. By grasping the types of bonds, their representations, and the properties of substances formed from these bonds, you will be well-prepared for your A-level exams. Consider further studying specific compounds and their applications to solidify your knowledge and enhance your exam performance.