Covalent bonding
Covalent bonds form between non-metal atoms, which share pairs of electrons instead of transferring them. The bonds themselves are strong, but how a substance behaves depends on whether it is made of small molecules or one giant structure.
Part 1 of 3: Learn it
In short
- A covalent bond is a shared pair of electrons between two non-metal atoms.
- Simple molecular substances have low melting points: only the weak attractions between separate molecules need overcoming.
- Giant covalent structures (diamond, graphite, silicon dioxide) have very high melting points.
Where this is in your specification
Spec points: AQA 4.2.1.4, 4.2.2.4, 4.2.2.6 and 4.2.3 (Combined Trilogy 5.2.1.4 and 5.2.3), Edexcel Topic 1, OCR C2.2
| Board | Topic: Bonding, structure and the properties of matter |
|---|---|
| AQA 8462 | 4.2 (8462); 5.2 (8464) |
| Edexcel 1CH0 | Topic 1 |
| OCR J248 | C2.2 |
| Eduqas C410 | 5 |
| Cambridge IGCSE 0620 | 2.4 to 2.7 |
| National 5 C813 75 | 1(b) Atomic structure and bonding related to properties |
Shared pairs
Each atom shares enough electrons to fill its outer shell. Hydrogen forms 1 bond, oxygen 2, nitrogen 3 and carbon 4. A double bond is two shared pairs (as in O₂ and CO₂); a triple bond is three (as in N₂).
| Molecule | Formula | Bonds |
|---|---|---|
| Hydrogen | H₂ | one single bond |
| Water | H₂O | two O-H single bonds |
| Ammonia | NH₃ | three N-H single bonds |
| Methane | CH₄ | four C-H single bonds |
| Oxygen | O₂ | one double bond |
Simple molecules
Inside each molecule the covalent bonds are strong, but the forces between separate molecules (intermolecular forces) are weak. Melting or boiling only overcomes those weak forces, so the melting and boiling points are low and many are gases or liquids at room temperature. Larger molecules have stronger intermolecular forces, so their boiling points are higher.
Simple molecules do not conduct electricity, because they have no overall charge and no free electrons.
Giant covalent structures
- Diamond: each carbon bonded to four others. Very hard, very high melting point, does not conduct.
- Graphite: each carbon bonded to three others in layers. The layers slide over each other, so it is soft. Each carbon has one delocalised electron, which lets graphite carry a current.
- Silicon dioxide: a giant lattice of silicon and oxygen atoms, with a very high melting point.
- Graphene is a single layer of graphite; fullerenes such as C₆₀ are hollow carbon molecules.
How many covalent bonds does a nitrogen atom usually form?
Show the answer
Three.
Part 2 of 3: See it worked
Worked examples
Example 1
Explain why methane has a very low boiling point.
- Methane is made of small molecules
- Boiling pulls the molecules apart, so only the weak attractions between them need to be overcome
- Little energy is needed to do that
Answer: The strong covalent bonds stay intact; only the feeble attractions holding one molecule to the next are overcome, and that takes little energy.
Example 2
Graphite is a conductor and diamond is not. Explain the difference.
- In graphite each carbon atom forms three covalent bonds
- The fourth outer electron is delocalised and can move through the structure
- In diamond all four outer electrons are used in bonds, so none are free
Answer: Graphite has delocalised electrons that can move and carry charge; diamond has none.
Common mistakes
- Saying covalent bonds break when a simple molecular substance boils. The intermolecular forces are overcome instead.
- Calling the bonds in simple molecules weak. The bonds are strong; the forces between molecules are weak.
- Saying graphite conducts because of ions. It is the delocalised electrons.
- Giving carbon the wrong number of bonds: it always forms four.
Why does a simple molecular substance not conduct electricity?
Show the answer
The molecules are neutral, and nothing charged is free to move through the substance.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
How many covalent bonds does a nitrogen atom usually form?
Three.
Why does a simple molecular substance not conduct electricity?
The molecules are neutral, and nothing charged is free to move through the substance.
Why is graphite soft and slippery?
Its layers are held together only by weak forces, so they can slide over each other.
Jobs that use this
- Chemist (se abre en otra pestaña)
- Chemical engineer (se abre en otra pestaña)
- Pharmacist (se abre en otra pestaña)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
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