2.2.2Additional lessons
Lewis Formulas — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 23 of 98
Part 1 of 6Drawing a Lewis Formula
A Lewis formula shows every valence electron as either a bonding pair or a lone pair.
1
Count the total valence electrons by adding those from each atom and adjusting for any ion charge.
2
Place the least electronegative atom (never hydrogen) in the centre and join the others to it with single bonds.
3
Give the outer atoms enough lone pairs to complete their octets, or two electrons for hydrogen.
4
Place any remaining electrons on the central atom as lone pairs.
5
If the central atom lacks an octet, convert lone pairs from outer atoms into double or triple bonds where appropriate.
Part 2 of 6Bonding and Lone Pairs
Bonding pair
- A pair of electrons shared between two atoms is shown as a line or two dots between their symbols.
Lone pair
- A non-bonding pair of outer electrons belongs to one atom and is shown as two dots on that atom.
Water
- H₂O has two bonding pairs and two lone pairs on oxygen, so oxygen has an octet and each hydrogen has two electrons.
Ammonia
- NH₃ has three bonding pairs and one lone pair on nitrogen, giving nitrogen an octet.
Methane
- CH₄ has four bonding pairs and no lone pairs, so every electron on carbon is shared.
| Molecule | Valence electrons | Bonding pairs | Lone pairs (total) |
|---|---|---|---|
| CH₄ | 8 | 4 | 0 |
| NH₃ | 8 | 3 | 1 |
| H₂O | 8 | 2 | 2 |
| HCl | 8 | 1 | 3 |
Part 3 of 6Exceptions to the Octet
Exceptions to the Octet
Electron deficient
- BF₃ has only six electrons around boron because boron has three valence electrons to share.
Expanded octet
- Elements in period 3 and below, such as P in PCl₅ or S in SF₆, can hold more than eight electrons.
Odd electron
- Molecules such as NO with an odd total number of valence electrons must have an unpaired electron.
Check charge
- For an ion, add one electron for each negative charge and remove one for each positive charge before drawing.
Part 4 of 6Checking Your Answer
Electron total
- The number of dots plus bonding electrons must equal the valence electron total counted at the start.
Octet check
- Outer atoms of second-period elements must have no more than eight electrons.
Hydrogen check
- Hydrogen never carries a lone pair and always forms exactly one bond.
Terminal atoms
- Halogen atoms bonded to a central atom usually have three lone pairs and one bonding pair.
Part 5 of 6Worked Example
Question Draw the Lewis formula of phosphorus trichloride, PCl₃.
- Count valence electronsP gives 5 and each Cl gives 7, so 5 + 3 × 7 = 26 electrons, which is 13 pairs.
- Put P in the centre with three single bonds to Cl, using 3 pairs.
- Add three lone pairs on each Cl, using 9 pairs, which leaves 1 pair.
- Place the last pair on phosphorus as a lone pair, giving every atom an octet.
Answer
- P is bonded to three Cl atoms and carries one lone pair;
- each Cl has three lone pairs.
Part 6 of 6Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Lewis formula A diagram showing all valence electrons of a molecule as bonding pairs and lone pairs.
- Valence electrons The electrons in the outermost shell of an atom.
- Lone pair A pair of valence electrons not involved in bonding.
- Expanded octet A central atom with more than eight electrons in its valence shell.
Common Mistakes
- Forgetting to add or subtract electrons for the charge on an ion.
- Giving hydrogen more than two electrons.
- Putting the most electronegative atom in the centre instead of the least electronegative.
- Writing lone pairs as shared pairs, which changes the number of bonds.
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