1.4.6Physical Chemistry
Covalent Dot-and-Cross Diagrams — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 15 of 42
Part 1 of 4Drawing Method
Dot-and-cross diagrams show only the outer shell electrons of each atom.
1
Count the outer electrons of each atom, adjusting for any ionic charge.
2
Draw the central atom and surround it with the other atoms, with overlapping outer shells where a bond forms.
3
Show shared pairs in the overlap using a dot from one atom and a cross from the other.
4
Add lone pairs so that each atom has a full outer shell where possible, usually eight.
5
Check the total electron count against your original count.
6
Where the molecule has a double or triple bond, share two or three pairs between the same two atoms before adding lone pairs to the remaining spaces.
Part 2 of 4Common Molecules
Water
- Oxygen shares one pair with each hydrogen and has two lone pairs: eight electrons around O and two around each H.
Ammonia
- Nitrogen shares one pair with each of three hydrogens and keeps one lone pair.
Carbon dioxide
- Carbon forms two double bonds with oxygen atoms, each oxygen also having two lone pairs.
Nitrogen
- Two nitrogen atoms share three pairs (a triple bond), each with one lone pair.
Hydrogen chloride
- Hydrogen shares one pair with chlorine, which keeps three lone pairs, giving two electrons around H and eight around Cl.
Part 3 of 4Special Cases
Special Cases
Dative bonding
- In NH₄⁺, the nitrogen lone pair in NH₃ is donated into an empty orbital of H⁺, forming a dative bond shown with an arrow.
Electron deficient
- Boron trifluoride, BF₃, has only six electrons around boron because boron has three outer electrons to share.
Expanded octet
- Elements in Period 3 and below, such as phosphorus in PCl₅ and sulfur in SF₆, can have more than eight outer electrons.
Ions
- A negative ion has extra electrons added to the count and a positive ion has fewer.
| Molecule | Bonding pairs | Lone pairs on central atom |
|---|---|---|
| CH₄ | 4 | 0 |
| NH₃ | 3 | 1 |
| H₂O | 2 | 2 |
| BF₃ | 3 | 0 |
| SF₆ | 6 | 0 |
Part 4 of 4Exam Checklist & Tips
1.4.6 – Key Points for the Exam
- Octet Eight electrons in the outer shell, giving a stable configuration.
- Expanded octet More than eight outer electrons around a central atom from Period 3 or below.
- Electron deficient Having fewer than eight electrons around the central atom, as in BF₃.
- Bonding pair A shared pair of electrons between two atoms.
Common Mistakes
- Forgetting the lone pairs on atoms such as oxygen, nitrogen and chlorine.
- Counting the wrong number of outer electrons for an ion.
- Showing both electrons of an ordinary covalent pair with the same symbol, instead of one dot and one cross.
- Assuming every atom must have eight outer electrons, which is untrue for BF₃, PCl₅ and SF₆.
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