Covalent Bonding

This lesson covers covalent bonding, the second major type of chemical bonding in the AQA GCSE Chemistry specification (4.2.2). Covalent bonding occurs between non-metal atoms and involves the sharing of electron pairs. You need to understand how covalent bonds form, be able to draw dot-and-cross diagrams for key molecules, and recognise single, double, and triple bonds.

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What Is Covalent Bonding?

A covalent bond is a shared pair of electrons between two atoms. Covalent bonding occurs between non-metal atoms. Each atom contributes one electron to the shared pair, and both atoms count the shared pair as part of their outer shell. This allows both atoms to achieve a full outer shell of electrons.

The shared pair of electrons is attracted to the nuclei of both atoms — this electrostatic attraction holds the atoms together.

Exam Tip: The definition of a covalent bond is: "A shared pair of electrons between two atoms." You must say "shared pair" not just "shared electrons." If you say "the atoms share electrons" without specifying "a pair," you may not get the mark.

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Why Do Covalent Bonds Form?

Covalent bonds form because:

1. Non-metal atoms need to gain electrons to achieve a full outer shell.
2. Instead of transferring electrons (as in ionic bonding), non-metal atoms share electrons.
3. Each shared pair counts towards the outer shell of both atoms.
4. The resulting molecule is more energetically stable than the separate atoms.

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Dot-and-Cross Diagrams for Covalent Molecules

You must be able to draw dot-and-cross diagrams for the following molecules. In these diagrams, use dots for the electrons of one atom and crosses for the electrons of the other.

Hydrogen (H2)

• Each hydrogen atom has 1 electron and needs 2 to fill its first shell.
• The two atoms share one pair of electrons — a single bond.
• Each hydrogen atom now has 2 electrons in its outer shell (like helium).

Chlorine (Cl2)

• Each chlorine atom has 7 outer electrons and needs 8 for a full outer shell.
• The two atoms share one pair of electrons — a single bond.
• Each chlorine atom now has 8 electrons in its outer shell (like argon).

Water (H2O)

• Oxygen has 6 outer electrons and needs 8.
• Each hydrogen has 1 outer electron and needs 2.
• Oxygen shares one pair of electrons with each of the two hydrogen atoms — forming two single bonds.
• Oxygen has two lone pairs (pairs of electrons not involved in bonding).

Ammonia (NH3)

• Nitrogen has 5 outer electrons and needs 8.
• Each hydrogen has 1 outer electron and needs 2.
• Nitrogen shares one pair of electrons with each of three hydrogen atoms — forming three single bonds.
• Nitrogen has one lone pair.

Methane (CH4)

• Carbon has 4 outer electrons and needs 8.
• Each hydrogen has 1 outer electron and needs 2.
• Carbon shares one pair of electrons with each of four hydrogen atoms — forming four single bonds.
• Carbon has no lone pairs.

Hydrogen Chloride (HCl)

• Hydrogen has 1 outer electron and needs 2.
• Chlorine has 7 outer electrons and needs 8.
• They share one pair of electrons — a single bond.

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Summary Table of Key Molecules

Molecule	Formula	Number of Shared Pairs	Bond Type	Lone Pairs on Central Atom
Hydrogen	H2	1	Single	0
Chlorine	Cl2	1	Single	3 per Cl
Water	H2O	2	Two single bonds	2 on O
Ammonia	NH3	3	Three single bonds	1 on N
Methane	CH4	4	Four single bonds	0 on C
Hydrogen chloride	HCl	1	Single	3 on Cl
Oxygen	O2	2	Double bond	2 per O
Nitrogen	N2	3	Triple bond	1 per N

Exam Tip: You must know the dot-and-cross diagrams for H2, Cl2, H2O, NH3, CH4, HCl, O2, and N2. These are all named in the specification and any one of them could appear on the exam paper. Practise drawing them until you can do them from memory.

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Double and Triple Bonds

Some molecules require more than one pair of shared electrons to achieve full outer shells.

Double Bond: Oxygen (O2)

• Each oxygen atom has 6 outer electrons and needs 8.
• Each atom needs to gain 2 electrons.
• The two oxygen atoms share two pairs of electrons — a double bond (shown as O=O).
• Each oxygen also has two lone pairs.

Triple Bond: Nitrogen (N2)

• Each nitrogen atom has 5 outer electrons and needs 8.
• Each atom needs to gain 3 electrons.
• The two nitrogen atoms share three pairs of electrons — a triple bond (shown as N≡N).
• Each nitrogen also has one lone pair.

graph TD
    A["Covalent Bonds"] --> B["Single Bond"]
    A --> C["Double Bond"]
    A --> D["Triple Bond"]
    B --> E["1 shared pair<br/>e.g. H2, Cl2, HCl, H2O"]
    C --> F["2 shared pairs<br/>e.g. O2, CO2"]
    D --> G["3 shared pairs<br/>e.g. N2"]
    E --> H["Bond Strength<br/>Increases"]
    F --> H
    G --> H

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e74c3c,color:#fff

Exam Tip: Double bonds are stronger and shorter than single bonds. Triple bonds are stronger and shorter than double bonds. If asked to compare bond strength, remember: triple > double > single. This also means that molecules with triple bonds (like N2) are very unreactive because a lot of energy is needed to break the strong triple bond.

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Carbon Dioxide (CO2)

Carbon dioxide is an important molecule with two double bonds.