2.2.2Additional lessons
Lewis Formulas — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 26 of 172
Part 1 of 6Parts of a Lewis Formula
A Lewis formula shows every valence electron pair in a molecule or polyatomic ion.
Bonding pair
- A shared pair of electrons, drawn as a line between two atomic symbols.
Lone pair
- A pair of valence electrons on one atom that is not involved in bonding, drawn as two dots.
Valence electrons
- The outer-shell electrons, equal to the group number minus 10 for groups 13 to 18.
Complete shells
- Hydrogen needs two electrons and most period 2 atoms need eight in their formulas.
Part 2 of 6Drawing Method
1
Add up the valence electrons of all atoms, adding one for each negative charge and subtracting one for each positive charge.
2
Choose the central atom, usually the least electronegative, and never hydrogen.
3
Join the outer atoms to the centre with single bonds, using two electrons for each.
4
Place the remaining electrons as lone pairs on the outer atoms first, until each has a full octet.
5
Put leftover electrons on the central atom, and form double or triple bonds if its octet is incomplete.
Part 3 of 6Examples
Examples
Water
- H₂O has 8 valence electrons: two bonding pairs and two lone pairs on oxygen.
Ammonia
- NH₃ has 8 valence electrons: three bonding pairs and one lone pair on nitrogen.
Ammonium ion
- NH₄⁺ has 5 + 4 − 1 = 8 electrons, all in four bonding pairs.
Hydrogen chloride
- HCl has one bonding pair and three lone pairs on chlorine.
| Species | Valence electrons | Lone pairs on central atom |
|---|---|---|
| CH₄ | 8 | 0 |
| NH₃ | 8 | 1 |
| H₂O | 8 | 2 |
Part 4 of 6Exceptions to the Octet
Electron-deficient
- Boron in BF₃ has only six electrons in its outer shell and is stable.
Odd-electron species
- NO has eleven valence electrons, so one atom cannot reach an octet.
Expanded octet
- Elements from period 3 downwards can have more than eight electrons, as covered in a later lesson.
Part 5 of 6Worked Example
Question Draw the Lewis formula of phosphorus trichloride, PCl₃.
- Valence electrons5 + 3 × 7 = 26.
- Phosphorus is central; three single bonds use 6 electrons.
- Place 3 lone pairs on each chlorine (18 electrons), leaving 26 − 6 − 18 = 2 electrons.
- These 2 electrons form one lone pair on phosphorus.
Answer
- Three P–Cl bonding pairs and one lone pair on P;
- every atom has eight electrons.
Part 6 of 6Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Lewis formula A diagram showing all bonding pairs and lone pairs of valence electrons.
- Lone pair A non-bonding pair of valence electrons on an atom.
- Valence electrons Electrons in the outermost shell of an atom.
- Electron deficient A species with fewer than eight electrons around the central atom.
Common Mistakes
- Forgetting to adjust the electron total for the charge on an ion.
- Placing hydrogen as the central atom.
- Giving a lone pair to each atom but leaving the central atom short of an octet.
- Counting the bond lines twice in the electron total.
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