# Covalent Bonding

> Edexcel International GCSE Chemistry · 4CH1
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-covalent-bonding/

This guide covers all core covalent bonding content for Edexcel IGCSE Chemistry (4CH1) specification points 1.44–1.51, including dot-cross diagrams, structure-property relationships, and giant covalent carbon allotropes.

**Prerequisites:** [Electronic configuration of atoms](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-electronic-configuration/); [Atomic structure and the periodic table](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-atomic-structure/)

## Learning objectives

- Define covalent bonding as the electrostatic attraction between shared electron pairs and atomic nuclei
- Draw dot-and-cross diagrams for specified diatomic, inorganic and simple organic covalent molecules
- Explain the relationship between structure and melting/boiling points for simple molecular and giant covalent substances
- Relate the structures of diamond, graphite and C60 fullerene to their physical properties
- Explain why most covalent compounds do not conduct electricity

## What is Covalent Bonding?

**Covalent bond** — A strong chemical bond formed when two non-metal atoms share a pair of electrons. The bond is held together by electrostatic attraction between the negatively charged shared electrons and the positively charged nuclei of the two bonded atoms.

Covalent bonds form between non-metal atoms only, as both atoms need to gain electrons to complete their outer electron shells. Atoms can share one, two or three pairs of electrons to form single, double or triple covalent bonds respectively.

**Worked example:** Describe the formation of a covalent bond in a hydrogen molecule (H₂)

1. 1. Each hydrogen atom has 1 electron in its outer (first) shell, which needs 1 more electron to be full and stable.
2. 2. Two hydrogen atoms come close together so their electron shells overlap, and they share their single outer electron, forming one shared pair of electrons.
3. 3. The negatively charged shared pair is attracted to the positively charged nucleus of both hydrogen atoms, creating a strong single covalent bond that holds the molecule together.

> **Exam tip:** Always explicitly mention electrostatic attraction between shared electrons and positive nuclei if asked to define or explain covalent bonding, as this is a required marking point.

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## Dot-and-Cross Diagrams for Covalent Molecules

Dot-and-cross diagrams are used to represent covalent bonding, with dots for electrons from one atom and crosses for electrons from the other bonded atom. Follow these rules for Edexcel IGCSE answers:

1. Only draw outer shell electrons for all atoms
2. Draw shared electron pairs in the overlapping area between the two bonded atoms
3. Include lone pairs of electrons (unshared outer electrons) on all atoms
4. Draw double bonds as two shared pairs, triple bonds as three shared pairs

**Worked example:** Draw a dot-and-cross diagram for a carbon dioxide molecule (CO₂)

1. 1. Carbon has 4 outer electrons, each oxygen has 6 outer electrons. Carbon needs 4 more electrons to fill its outer shell, each oxygen needs 2 more.
2. 2. Carbon shares 2 electrons with each oxygen atom, forming two double covalent bonds (one between C and each O atom).
3. 3. Draw the C nucleus in the centre, with an O nucleus on either side. Show two shared pairs of electrons in each overlap zone between C and O.
4. 4. Add the remaining 4 outer electrons (two lone pairs) to each oxygen atom, outside the overlap zone.

> **Exam tip:** You must draw double bonds for O₂, CO₂ and ethene, and a triple bond for N₂. These are frequently tested, so memorise their dot-cross structures.

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## Simple Molecular Substances

Simple molecular substances are made of small, discrete molecules held together by weak intermolecular forces between molecules. Strong covalent bonds only exist *inside* each molecule, holding the atoms of the molecule together.

- Low melting/boiling points: Weak intermolecular forces require very little energy to overcome, so many simple molecular substances are gases, liquids or soft solids at room temperature
- Melting/boiling points increase with increasing relative molecular mass (Mᵣ): Larger molecules have stronger intermolecular forces between them, so more energy is needed to overcome these forces
- Do not conduct electricity: All electrons are fixed in covalent bonds or lone pairs, so there are no free charged particles to carry charge

**Worked example:** Explain why methane (CH₄, Mᵣ=16) has a lower boiling point than octane (C₈H₁₈, Mᵣ=114)

1. 1. Both methane and octane are simple molecular substances, held together by weak intermolecular forces between molecules.
2. 2. Octane has a much higher relative molecular mass than methane, so the intermolecular forces between octane molecules are stronger than those between methane molecules.
3. 3. More energy is required to overcome the stronger intermolecular forces in octane, so octane has a higher boiling point than methane.

> **Exam tip:** Never state that covalent bonds are broken when melting or boiling simple molecular substances. This is one of the most common marking point losses for this topic, so always explicitly reference weak intermolecular forces being overcome.

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## Giant Covalent Structures

Giant covalent structures are large, continuous 3D lattices where every atom is linked to adjacent atoms by strong covalent bonds. You need to recall the structure and properties of three carbon allotropes (different forms of the same element) for your exam: diamond, graphite and C60 fullerene.

- Very high melting/boiling points: A huge number of strong covalent bonds must be broken to melt the structure, which requires a very large amount of energy

**Worked example:** Compare the hardness and electrical conductivity of diamond and graphite, linking your answer to their structures

1. 1. Hardness: Diamond has a tetrahedral lattice where each C atom is bonded to 4 other C atoms, with no weak points, so it is extremely hard. Graphite is made of flat hexagonal layers held together by weak intermolecular forces, so layers can slide over each other easily, making graphite soft and slippery.
2. 2. Electrical conductivity: Each C atom in diamond uses all 4 outer electrons in covalent bonds, so there are no free electrons, so diamond does not conduct. Each C atom in graphite only forms 3 covalent bonds, leaving one delocalised free electron per C atom that can move through the structure, so graphite conducts electricity.

> **Exam tip:** C60 fullerene has a lower melting point than diamond and graphite, as it is made of discrete spherical molecules held together by weak intermolecular forces, not a continuous 3D covalent lattice.

*Calculator:* forbidden

## Electrical Conductivity of Covalent Compounds

Almost all covalent compounds do not conduct electricity in any state. This is because all electrons are held in fixed covalent bonds or as lone pairs on atoms, so there are no free charged particles (either electrons or ions) that can move to carry electrical charge.

> **Only exception**
>
> Graphite is the only covalent substance you need to recall that conducts electricity, due to its delocalised free electrons between layers.

**Worked example:** Explain why solid hydrogen chloride (HCl) does not conduct electricity, but solid graphite does

1. 1. Solid HCl is a simple molecular substance. All electrons are fixed in covalent bonds between H and Cl atoms, and there are no free charged particles, so it cannot conduct electricity.
2. 2. Graphite is a giant covalent substance with delocalised free electrons that can move through the structure to carry electrical charge, so it conducts electricity even when solid.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Stating covalent bonds are broken when melting/boiling simple molecular substances
  - Why it fails: Confuses strong intramolecular covalent bonds inside molecules with weak intermolecular forces between separate molecules, which is a key marking discriminator
  - Correct: Explicitly state that only weak intermolecular forces are overcome when melting/boiling simple molecular substances, and covalent bonds remain intact
- **Wrong:** Drawing inner shell electrons in dot-and-cross diagrams
  - Why it fails: Edexcel mark schemes only require outer shell electrons to be shown, and extra inner shells may cost marks if drawn incorrectly
  - Correct: Only draw outer shell electrons for all atoms in covalent dot-cross diagrams, including lone pairs
- **Wrong:** Stating C60 fullerene conducts electricity
  - Why it fails: Unlike graphite, C60 has no free delocalised electrons that can move across the structure to carry charge
  - Correct: Only state that graphite conducts electricity of the three carbon allotropes required for the specification
- **Wrong:** Drawing single bonds for O₂, CO₂ or ethene
  - Why it fails: These molecules contain double covalent bonds, which are required for correct dot-cross representations to meet the octet rule for all atoms
  - Correct: Draw two shared pairs of electrons in the overlap zone for double bonds in O₂, CO₂ and ethene, and three shared pairs for the triple bond in N₂
- **Wrong:** Attributing graphite's softness to weak covalent bonds
  - Why it fails: Covalent bonds within graphite's hexagonal layers are very strong; its softness comes from weak forces between layers
  - Correct: State that graphite is soft because weak intermolecular forces between its layers allow layers to slide over each other easily

## Cheatsheet

| Structure Type | Key Features | Melting/Boiling Point | Electrical Conductivity | Examples |
| --- | --- | --- | --- | --- |
| Simple molecular | Small discrete molecules, weak intermolecular forces between molecules | Low | Does not conduct | H₂, NH₃, CO₂, methane |
| Giant covalent | Continuous lattice of atoms linked by strong covalent bonds | Very high | Does not conduct (except graphite) | Diamond, graphite, C60 fullerene |
| Diamond | Tetrahedral lattice, each C bonded to 4 other C atoms | Extremely high | Does not conduct | — |
| Graphite | Layered hexagonal lattice, each C bonded to 3 other C atoms, weak interlayer forces | Extremely high | Conducts electricity | — |
| C60 fullerene | Spherical molecule of 60 C atoms, weak intermolecular forces between molecules | Lower than diamond/graphite | Does not conduct | — |

## What's next

Now you have mastered core covalent bonding content, you are ready to move on to related topics in the Principles of Chemistry unit. Next, you will learn about ionic bonding, which forms between metal and non-metal atoms via electron transfer, and compare the structure and properties of ionic compounds to the covalent substances you have studied here. You will also apply your understanding of structure-property relationships when you cover electrolysis later in the unit, where you will explore how different substances conduct electricity when molten or dissolved in water. Covalent bonding is also foundational for organic chemistry, which you will encounter later in the course, as all organic molecules are held together by covalent bonds. Practice drawing dot-cross diagrams regularly and memorise the structure-property links for giant covalent structures to maximise your marks in exam questions.

- [Ionic Bonding](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-ionic-bonding/)

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