🔗 Chemical Bonding
Secondary School Chemistry • MalawiHub
1. Introduction
A chemical bond is the force that holds atoms or ions together in a substance.
Atoms form bonds because bonded arrangements are generally more stable than isolated atoms.
2. Learning Objectives
- Explain why atoms form chemical bonds.
- Describe ionic, covalent and metallic bonding.
- Draw simple electron-dot structures.
- Relate bonding to physical properties.
- Answer examination questions involving chemical bonding.
3. Ionic Bonding
An ionic bond is the strong electrostatic attraction between oppositely charged ions.
It normally forms when electrons are transferred from a metal atom to a non-metal atom.
Sodium loses one electron:
Na → Na⁺ + e⁻
Chlorine gains one electron:
Cl + e⁻ → Cl⁻
The ions then attract:
Na⁺ + Cl⁻ → NaCl
Properties of ionic compounds
- Usually have high melting and boiling points.
- Usually form crystalline solids.
- Conduct electricity when molten or dissolved in water.
- Do not usually conduct electricity when solid.
- Many are soluble in water.
4. Covalent Bonding
A covalent bond forms when atoms share pairs of electrons.
Covalent bonding usually occurs between non-metal atoms.
Types
| Bond | Shared electron pairs | Example |
|---|---|---|
| Single | 1 pair | H₂ |
| Double | 2 pairs | O₂ |
| Triple | 3 pairs | N₂ |
Each hydrogen atom contributes one electron. The two electrons are shared, forming H—H.
Simple molecular substances
Examples include H₂, O₂, CO₂ and CH₄. They generally have low melting and boiling points because the forces between their molecules are relatively weak.
Giant covalent structures
Diamond and graphite contain many atoms joined by strong covalent bonds.
5. Metallic Bonding
Metallic bonding is the attraction between positive metal ions and a sea of delocalised electrons.
Properties of metals
- Good electrical conductors.
- Good thermal conductors.
- Usually high melting points.
- Malleable and ductile.
- Usually shiny when freshly cut or polished.
6. Comparing Bonding Types
| Bonding | Particles involved | Main feature |
|---|---|---|
| Ionic | Positive and negative ions | Electron transfer |
| Covalent | Atoms | Electron sharing |
| Metallic | Metal ions and electrons | Delocalised electrons |
7. Examination Focus
Be able to:
- Define ionic, covalent and metallic bonding.
- Explain electron transfer in ionic compounds.
- Explain electron sharing in covalent compounds.
- Give properties of ionic and metallic substances.
- Explain why molten ionic compounds conduct electricity.
- Distinguish between a covalent molecule and an ion.
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
In solid sodium chloride, the ions are fixed in a crystal lattice and cannot move freely. When sodium chloride is molten, the ions become free to move and can carry electric charge.
8. Practice Exercises
- Define a chemical bond.
- What happens to sodium when it forms Na⁺?
- What type of bonding exists in NaCl?
- How many electron pairs are shared in a double bond?
- State two properties of ionic compounds.
- Why do metals conduct electricity?
- Distinguish between ionic and covalent bonding.
- Explain why graphite can conduct electricity.
Answers
- A force that holds atoms or ions together.
- It loses one electron.
- Ionic bonding.
- Two pairs.
- High melting point and electrical conduction when molten or dissolved.
- They contain mobile delocalised electrons.
- Ionic bonding involves electron transfer; covalent bonding involves electron sharing.
- Graphite contains delocalised electrons that can move through its structure.
9. Common Mistakes
- Do not say ionic bonding involves sharing electrons.
- Do not say solid ionic compounds conduct because their ions cannot move.
- Do not confuse an ion with an atom.
- Remember that covalent bonds involve sharing electrons.
10. Examination Tip
When explaining a property, always connect the property to the particles or bonding involved. This makes your answer more complete.