1.5.7Physical Chemistry
Covalent Structures — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 23 of 42
Part 1 of 5Covalent Bonding
Shared pairs
- A covalent bond is a shared pair of electrons attracted to the nuclei of both bonded atoms.
Single and multiple bonds
- Atoms can share one, two or three pairs, forming single, double and triple bonds with increasing strength.
Bond length
- More shared pairs bring the nuclei closer together, so triple bonds are shorter than double bonds, which are shorter than single bonds.
Lone pairs
- Outer-shell electrons not used in bonding stay as lone pairs and affect the shape of the molecule.
Part 2 of 5Dative Covalent Bonds
Dative Covalent Bonds
Definition
- A dative covalent bond forms when one atom provides both electrons of the shared pair.
Ammonium ion
- NH₃ donates its lone pair to a hydrogen ion H⁺, forming NH₄⁺ with four identical N–H bonds.
Oxonium ion
- A water molecule uses a lone pair to bond to H⁺, giving H₃O⁺.
Drawing
- Show the dative bond as an arrow pointing from the lone pair donor to the acceptor atom.
Part 3 of 5Beyond the Octet
Expanded octet
- Elements in period 3 and below, such as sulfur and phosphorus, can hold more than eight electrons in the outer shell.
PCl₅
- Phosphorus forms five bonds, so it has ten electrons around it.
SF₆
- Sulfur forms six bonds and has twelve electrons in its outer shell.
Incomplete octet
- Boron in BF₃ has only six electrons, making it electron deficient and able to accept a lone pair.
| Species | Bonding pairs around central atom | Electrons in outer shell |
|---|---|---|
| BF₃ | 3 | 6 |
| CH₄ | 4 | 8 |
| PCl₅ | 5 | 10 |
| SF₆ | 6 | 12 |
Part 4 of 5Drawing Dot-and-Cross Diagrams
1
Work out the number of outer-shell electrons for each atom from its group number.
2
Place the least electronegative atom in the centre, except for hydrogen, which is always terminal.
3
Share electron pairs between the central atom and the surrounding atoms to complete outer shells.
4
Add any remaining electrons as lone pairs on each atom.
5
For an ion, adjust the total number of electrons to fit the charge before drawing.
Part 5 of 5Exam Checklist & Tips
1.5.7 – Key Points for the Exam
- Covalent bond Shared pair of electrons between two atoms.
- Dative covalent bond A bond in which both shared electrons come from the same atom.
- Lone pair A pair of outer-shell electrons not involved in bonding.
- Expanded octet More than eight electrons in the outer shell of a period 3 or lower element.
Common Mistakes
- Drawing NH₄⁺ with one bond different from the others; once formed, all four N–H bonds are identical.
- Applying the octet rule to sulfur or phosphorus compounds where it does not hold.
- Forgetting to include the charge brackets when drawing an ion.
- Adding the dative arrow in the wrong direction; it points away from the donor.
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