Covalent Bonding
Edexcel International GCSE ChemistryΒ· 1.44β1.51Β· 25 min read
1. What is Covalent Bonding?β β ββββ± 5 min
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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.
Describe the formation of a covalent bond in a hydrogen molecule (Hβ)
- 1
- Each hydrogen atom has 1 electron in its outer (first) shell, which needs 1 more electron to be full and stable.
- 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
- 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.
2. Dot-and-Cross Diagrams for Covalent Moleculesβ β β βββ± 7 min
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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:
Only draw outer shell electrons for all atoms
Draw shared electron pairs in the overlapping area between the two bonded atoms
Include lone pairs of electrons (unshared outer electrons) on all atoms
Draw double bonds as two shared pairs, triple bonds as three shared pairs
Draw a dot-and-cross diagram for a carbon dioxide molecule (COβ)
- 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
- Carbon shares 2 electrons with each oxygen atom, forming two double covalent bonds (one between C and each O atom).
- 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
- 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 Oβ, COβ and ethene, and a triple bond for Nβ. These are frequently tested, so memorise their dot-cross structures.
3. Simple Molecular Substancesβ β β βββ± 6 min
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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 (Mα΅£): 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
Explain why methane (CHβ, Mα΅£=16) has a lower boiling point than octane (CβHββ, Mα΅£=114)
- 1
- Both methane and octane are simple molecular substances, held together by weak intermolecular forces between molecules.
- 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
- 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.
4. Giant Covalent Structuresβ β β β ββ± 7 min
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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
Compare the hardness and electrical conductivity of diamond and graphite, linking your answer to their structures
- 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
- 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.
5. Electrical Conductivity of Covalent Compoundsβ β ββββ± 3 min
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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.
Explain why solid hydrogen chloride (HCl) does not conduct electricity, but solid graphite does
- 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
- 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.
6. Common Pitfalls
Wrong move:
Stating covalent bonds are broken when melting/boiling simple molecular substances
Why:
Confuses strong intramolecular covalent bonds inside molecules with weak intermolecular forces between separate molecules, which is a key marking discriminator
Correct move:
Explicitly state that only weak intermolecular forces are overcome when melting/boiling simple molecular substances, and covalent bonds remain intact
Wrong move:
Drawing inner shell electrons in dot-and-cross diagrams
Why:
Edexcel mark schemes only require outer shell electrons to be shown, and extra inner shells may cost marks if drawn incorrectly
Correct move:
Only draw outer shell electrons for all atoms in covalent dot-cross diagrams, including lone pairs
Wrong move:
Stating C60 fullerene conducts electricity
Why:
Unlike graphite, C60 has no free delocalised electrons that can move across the structure to carry charge
Correct move:
Only state that graphite conducts electricity of the three carbon allotropes required for the specification
Wrong move:
Drawing single bonds for Oβ, COβ or ethene
Why:
These molecules contain double covalent bonds, which are required for correct dot-cross representations to meet the octet rule for all atoms
Correct move:
Draw two shared pairs of electrons in the overlap zone for double bonds in Oβ, COβ and ethene, and three shared pairs for the triple bond in Nβ
Wrong move:
Attributing graphite's softness to weak covalent bonds
Why:
Covalent bonds within graphite's hexagonal layers are very strong; its softness comes from weak forces between layers
Correct move:
State that graphite is soft because weak intermolecular forces between its layers allow layers to slide over each other easily
7. Quick Reference 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 | Hβ, NHβ, COβ, 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 | β |
Graphite | Layered hexagonal lattice, each C bonded to 3 other C atoms, weak interlayer forces | Extremely high | Conducts electricity | β |
C60 fullerene | Spherical molecule of 60 C atoms, weak intermolecular forces between molecules | Lower than diamond/graphite | Does not conduct | β |
8. Frequently Asked
Do I need to draw inner shell electrons in dot-cross diagrams for covalent molecules?
No, Edexcel IGCSE mark schemes only require outer shell electrons to be shown. Drawing inner shells is unnecessary and may cost marks if drawn incorrectly.
Why do simple molecular substances have low melting and boiling points?
Weak intermolecular forces between separate molecules require very little energy to overcome. You must explicitly state that strong covalent bonds inside molecules are not broken during melting/boiling for full marks.
Does C60 fullerene conduct electricity?
No, unlike graphite, C60 has no free delocalised electrons that can move through the structure to carry electrical charge. Only graphite is a conductor of the three carbon allotropes you need to recall.
Going deeper
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.