1.2Unit 2 · Bonding & Electrolysis
Covalent bonding — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 2 of 4 1 quiz
Explain how covalent bonds form.Covalent bonds form between non-metal atoms.Bonding pairs are attracted to both nuclei, holding the atoms together.Simple molecular substances have low m.p.
- Formation of covalent bonds
- Dot and cross diagrams
- Multiple bonds
- Simple molecular structures
- Giant covalent structures
- Covalent vs ionic
Section 1 of 4Covalent bonding
Formation of covalent bonds
- when non-metal atoms react with each other, they share outer electrons so each reaches a full shell.
- The shared pair is attracted electrostatically to both positive nuclei, which makes the bond strong.
Dot and cross diagrams
- H2 has one shared pair;
- H2O has two O–H bonds plus two lone pairs on O;
- CH4 has four C–H bonds;
- NH3 has three N–H bonds and one lone pair;
- HCl has one bond and three lone pairs on Cl.
- H2 has one shared pair;
Multiple bonds
O2 (O=O double bond), CO2 (two C=O double bonds), N2 (N≡N triple bond), C2H4 (one C=C double bond).Simple molecular structures
- strong covalent bonds within each molecule but weak intermolecular forces between molecules.
- Melting or boiling only overcomes the weak forces, so little energy is needed – many are gases or liquids at room temperature.
- Larger molecules have stronger intermolecular forces, so their boiling points are higher (e.g. down the halogens, or along the alkanes).
Giant covalent structures
- every atom is joined to others by covalent bonds in a continuous network.
- Melting requires breaking many strong bonds, so melting points are very high and they are insoluble in water.
Diamond
each carbon forms four bonds in a rigid tetrahedral network – very hard, no free electrons. Graphite: each carbon forms three bonds in hexagonal layers;- weak forces between layers let them slide (soft, lubricant);
- one delocalised electron per atom allows conduction. C60 is a simple molecule with weak intermolecular forces, so it is soft with a relatively low sublimation point.
Covalent vs ionic
- in a covalent bond, electrons are shared between non-metal atoms and no ions form.
- In ionic bonding, electrons are transferred from a metal to a non-metal, making oppositely charged ions.
Bonding and non-bonding electrons
- the shared electrons in a bond are a bonding pair.
- Outer electrons not used in bonds are non-bonding (lone) pairs.
- Usually each atom gives one electron to each bond, which is drawn as a line, e.g. Cl–Cl.
More dot and cross examples
- Cl2: one shared pair, and each Cl has three lone pairs.
- Ethane, C2H6: one C–C bond and six C–H bonds, no lone pairs.
- A double bond such as O=O shares four electrons.
Why bigger molecules boil higher
molecules with a larger relative molecular mass have more electrons, so the intermolecular attractions between them are stronger and more energy is needed to separate them.Graphite melting point
- graphite is soft because its layers slide, but it still has a very high melting point.
- Melting it means breaking the many strong covalent bonds within the layers.
C<sub>60</sub> structure
- buckminsterfullerene is a hollow sphere of 60 carbon atoms arranged in 12 pentagons and 20 hexagons.
- Fullerenes can carry drugs inside the cage, hold catalysts on their large surface and act as lubricants.
C<sub>60</sub> conductivity
- each carbon in C60 has a spare electron, but these electrons can only move within one molecule, not from molecule to molecule.
- So solid C60 does not conduct electricity, unlike graphite.
Covalent insulators
- having no free ions or electrons, simple covalent substances do not conduct even when molten.
- Plastics are used to coat electrical wires for this reason.
Section 2 of 4Process & sequence
- Write the number of outer electrons for each atom (e.g. H 1, O 6).
- Work out how many electrons each atom needs to fill its outer shell.
- Place atoms so their outer shells overlap.
- Put one shared pair (one dot and one cross) in each overlap for each bond needed.
- Add the remaining non-bonding electrons (lone pairs) to each atom.
- Check every atom has a full outer shell (H = 2, others = 8).
Drawing a covalent dot and crosswrite these steps in this order; each step is a marking point.
- Count outer electrons
- Find electrons needed
- Overlap outer shells
- Add shared pairs
- Add lone pairs
- Check full shells
Section 3 of 4Compare
| Property | Simple molecular | Giant covalent |
|---|---|---|
| Examples | H2O, CO2, CH4 | Diamond, graphite, SiO2 |
| Melting point | Low | Very high |
| What is overcome on melting | Weak intermolecular forces | Many strong covalent bonds |
| Conductivity | Does not conduct | Does not conduct (except graphite) |
| Allotrope | Structure | Property / use |
|---|---|---|
| Diamond | Each C bonded to 4 others, tetrahedral | Very hard → cutting tools |
| Graphite | Each C bonded to 3 others in layers; delocalised electrons; weak forces between layers | Soft, slippery, conducts → lubricant, electrodes |
| C60 fullerene | Simple molecule, 60 C atoms in a sphere | Low m.p., slippery; weak intermolecular forces |
Section 4 of 4Worked examples
Describe the bonding in a molecule of carbon dioxide.
- Carbon has 4 outer electrons and each oxygen has 6.
- Carbon shares two pairs with each oxygen, forming two C=O double bonds.
- Each O also has two lone pairs, so all atoms have 8 outer electrons.
- Carbon has 4 outer electrons and each oxygen has 6.
Explain why methane has a low boiling point but diamond has a very high melting point.
- Methane is simple molecular: boiling only overcomes weak intermolecular forces.
- Diamond is giant covalent: melting needs many strong covalent bonds to be broken, requiring much more energy.
- Methane is simple molecular: boiling only overcomes weak intermolecular forces.
Marked practiceQuizzes for this lesson
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