Covalent Bonding

This lesson covers covalent bonding — the sharing of electron pairs between non-metal atoms — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how covalent bonds form, draw dot-and-cross diagrams for simple molecules, and recognise common covalent substances.

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What Is Covalent Bonding?

A covalent bond is a shared pair of electrons between two non-metal atoms. Both atoms contribute one electron to the shared pair.

Covalent bonding occurs between non-metals only. It happens because non-metal atoms need to gain electrons to achieve a full outer shell, and they do this by sharing rather than transferring.

How a Covalent Bond Forms

1. Two non-metal atoms each have incomplete outer electron shells.
2. Their outer shells overlap.
3. A pair of electrons (one from each atom) is shared in the overlap region.
4. Both atoms now count the shared pair as part of their outer shell, achieving a full outer shell.
5. The shared electrons are attracted to the nuclei of both atoms — this attraction holds the atoms together.

Exam Tip: A covalent bond is a "shared pair of electrons." Always say pair — one electron alone does not form a covalent bond.

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Dot-and-Cross Diagrams for Covalent Molecules

Dot-and-cross diagrams show the outer-shell electrons around atoms in a molecule. Dots represent electrons from one atom, crosses from the other. The shared pair appears in the overlap.

Hydrogen (H₂)

• Each hydrogen atom has 1 electron.
• They share 1 pair — a single covalent bond.
• Each hydrogen atom now has 2 outer electrons (like helium).

Hydrogen Chloride (HCl)

• Hydrogen has 1 outer electron; chlorine has 7 outer electrons.
• They share 1 pair of electrons.
• Hydrogen achieves 2 outer electrons; chlorine achieves 8 outer electrons.

Water (H₂O)

• Oxygen has 6 outer electrons — it needs 2 more.
• Each hydrogen shares 1 electron with the oxygen.
• Oxygen forms 2 covalent bonds (one with each hydrogen).
• Oxygen has 2 lone pairs (pairs of electrons not involved in bonding).

Ammonia (NH₃)

• Nitrogen has 5 outer electrons — it needs 3 more.
• Each hydrogen shares 1 electron with the nitrogen.
• Nitrogen forms 3 covalent bonds with three hydrogen atoms.
• Nitrogen has 1 lone pair.

Methane (CH₄)

• Carbon has 4 outer electrons — it needs 4 more.
• Each hydrogen shares 1 electron with the carbon.
• Carbon forms 4 covalent bonds with four hydrogen atoms.
• Carbon has no lone pairs.

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Summary of Common Covalent Molecules

Molecule	Formula	Number of Covalent Bonds	Lone Pairs on Central Atom	Shape
Hydrogen	H₂	1 (single)	0	Linear
Chlorine	Cl₂	1 (single)	3 per Cl	Linear
Hydrogen chloride	HCl	1 (single)	3 on Cl	Linear
Water	H₂O	2 (single)	2 on O	Bent
Ammonia	NH₃	3 (single)	1 on N	Pyramidal
Methane	CH₄	4 (single)	0	Tetrahedral

graph TD
    A["Covalent Bonding"] --> B["Between non-metals"]
    A --> C["Shared pairs of electrons"]
    A --> D["Atoms achieve full<br/>outer shells"]
    C --> E["Single bond = 1<br/>shared pair"]
    C --> F["Double bond = 2<br/>shared pairs"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#1a1a2e,color:#fff
    style F fill:#1a1a2e,color:#fff

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Double Covalent Bonds

Some atoms need to share two pairs of electrons to achieve a full outer shell. This forms a double covalent bond, shown as a double line (=) in displayed formulae.

Oxygen (O₂)

• Each oxygen atom has 6 outer electrons — it needs 2 more.
• The two oxygen atoms share 2 pairs of electrons — a double bond.
• Each oxygen now has 8 outer electrons (full outer shell).

Carbon Dioxide (CO₂)

• Carbon has 4 outer electrons; each oxygen has 6 outer electrons.
• Carbon forms a double bond with each of the two oxygen atoms.
• Carbon shares 4 pairs of electrons in total (2 double bonds).
• Structure: O=C=O.

Exam Tip: In a double bond, 2 pairs of electrons are shared. Make sure your dot-and-cross diagram shows 4 electrons (2 dots and 2 crosses) in each overlap region for a double bond.

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Drawing Dot-and-Cross Diagrams — Rules

1. Show only the outer-shell electrons (unless told otherwise).
2. Use dots for electrons from one atom and crosses for the other.
3. Shared pairs should contain one dot and one cross.
4. Show lone pairs (non-bonding pairs) as well.
5. Make sure each atom achieves a full outer shell (2 for hydrogen, 8 for other atoms in Period 2 and 3).

Exam Tip: A common mistake is to forget lone pairs. Oxygen has 2 lone pairs in water, and nitrogen has 1 lone pair in ammonia. You must show these.

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Summary

• A covalent bond is a shared pair of electrons between two non-metal atoms.
• Atoms share electrons to achieve a full outer shell (stable noble gas configuration).
• Dot-and-cross diagrams show shared and lone pairs of electrons in molecules.
• A single bond is 1 shared pair; a double bond is 2 shared pairs.
• Common molecules include H₂, Cl₂, HCl, H₂O, NH₃, CH₄, O₂ and CO₂.

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Worked Examples — Reasoning About Dot-and-Cross Diagrams

Example 1 — Chlorine (Cl₂)

Each chlorine atom has the electron configuration 2, 8, 7. It needs one more electron to reach 2, 8, 8 (argon configuration). Neither atom will transfer — they are both non-metals with high electronegativity — so instead they each contribute one electron to a shared pair.

• Draw one Cl atom with 7 outer dots, the other with 7 outer crosses.
• In the overlap: one dot from the first atom pairs with one cross from the second.
• Around each Cl atom: three lone pairs (6 non-bonding electrons) plus the shared pair.
• Each Cl now counts 8 outer electrons.