1.4.5Physical Chemistry
Covalent Bonding Overview — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 14 of 42
Part 1 of 4The Shared Pair
Sharing electrons lets both atoms effectively fill their outer shells without forming ions.
Definition
- A covalent bond is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the two bonded atoms.
Who forms it
- Non-metal atoms share electrons so that each gains a more stable, full outer shell.
Bonding and lone pairs
- Pairs in the bond are bonding pairs, while pairs in the outer shell not involved in bonding are lone pairs.
Strength
- The attraction is strong, so covalent bonds need a large amount of energy to break.
Electron sharing
- Hydrogen needs only two electrons to fill its first shell,
- so a hydrogen molecule has a single shared pair even though it has no octet.
Part 2 of 4Types of Covalent Bond
Types of Covalent Bond
Single bond
- One shared pair, as in H₂, Cl₂ or the C–H bonds in methane.
Double bond
- Two shared pairs, as in O₂ and CO₂, which is shorter and stronger than a single bond.
Triple bond
- Three shared pairs, as in N₂, which is the shortest and strongest.
Dative covalent bond
- Both electrons of the shared pair come from one atom, as in the ammonium ion NH₄⁺ formed from NH₃ and H⁺.
Ethene
- Ethene contains a C=C double bond, which makes it a more reactive molecule than ethane because its electron density is more exposed.
| Bond | Shared pairs | Example |
|---|---|---|
| Single | 1 | H–H |
| Double | 2 | O=O |
| Triple | 3 | N≡N |
| Dative | 1 (from one atom) | N→H in NH₄⁺ |
Part 3 of 4Structures
1
Simple molecular substances contain small, separate molecules held together by strong covalent bonds inside each molecule.
2
Between simple molecules only weak intermolecular forces act, so they have low melting and boiling points.
3
Giant covalent structures such as diamond and silicon dioxide have atoms bonded by covalent bonds throughout the whole lattice.
4
Breaking a giant covalent lattice means breaking many strong covalent bonds, so melting points are very high.
5
Simple molecules do not conduct electricity because they have no charged particles that are free to move.
Part 4 of 4Exam Checklist & Tips
1.4.5 – Key Points for the Exam
- Covalent bond The electrostatic attraction between a shared pair of electrons and two nuclei.
- Lone pair A pair of outer-shell electrons not involved in bonding.
- Dative covalent bond A covalent bond in which both shared electrons come from the same atom.
- Giant covalent lattice A structure in which covalent bonds extend throughout the solid.
Common Mistakes
- Saying that covalent bonds are broken when a simple molecular substance melts;
- only intermolecular forces are overcome.
- Describing the covalent bond as the attraction between the two nuclei.
- Drawing the dative bond as if both atoms had supplied one electron.
- Assuming all covalent substances have low melting points, which is untrue for giant covalent structures.
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