Compound Structure and Properties
Lewis Diagrams — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 9 parts Lesson 3 of 13 Unit quiz
Lesson summary
Lewis Diagrams — AP CHEMISTRY notes: Draw Lewis diagrams, use resonance and formal charge to choose structures, and recognise the limitations of the model.A Lewis diagram shows valence electrons around atoms, with bonding pairs as lines and nonbonding (lone) pairs as dots.Double …
- Octet rule
- Exceptions
- Resonance structures
- Nitrate has 5 + 3(6) + 1
- BH4−: boron has 3 − 0 − 4
- Resonance hybrid
Section 1 of 9Lewis diagrams
A Lewis diagram shows valence electrons around atoms, with bonding pairs as lines and nonbonding (lone) pairs as dots.
Double bonds are two lines (four shared electrons) and triple bonds three lines (six shared electrons).
Octet rule
Octet rule atoms are most stable with eight valence electrons.
Exceptions
- Exceptions hydrogen completes a duet and boron can have a sextet.
Section 2 of 9Drawing a Lewis diagram
1
Count the total valence electrons, adding one per negative charge and subtracting one per positive charge.
2
Draw a skeletal structure showing how atoms are linked.
3
Place one pair between each pair of linked atoms.
4
Add lone pairs to complete octets, starting with terminal atoms and finishing with the central atom.
5
If an atom lacks an octet, form double or triple bonds from lone pairs.
6
Check that every atom has an octet (hydrogen a duet) and that the electron total matches.
| Molecule | Valence electrons |
|---|---|
| CH4 | 4 + 4(1) = 8 |
| NH3 | 5 + 3(1) = 8 |
| H2O | 2(1) + 6 = 8 |
| CO2 | 4 + 2(6) = 16 |
| HCN | 1 + 4 + 5 = 10 |
Section 3 of 9Checking a diagram
Checking a diagram
A nitrogen atom with only six electrons (two bonding pairs and one lone pair) is drawn incorrectly because its octet is incomplete.
Boron in BF3 is stable with a sextet, halogens complete octets and hydrogen completes a duet.
Section 4 of 9Resonance
Resonance
Resonance structures
Lewis structures of the same molecule that differ only in the arrangement of electrons.
The resonance hybrid is the actual structure, a mixture of all resonance structures.
Resonance arises because electrons are delocalised, spread over several atoms.
The hybrid is more stable than any single resonance structure and is drawn with dotted lines.
Examples include the nitrate ion, carbonate ion, benzene, ozone and the carboxylate ion.
Nitrate has 5 + 3(6) + 1
24 valence electrons and three equivalent structures.
Section 5 of 9Formal charge
Formal charge
V − N − B/2, where V is valence electrons, N nonbonding electrons and B bonding electrons.
Formal charge assumes bonding electrons are shared equally, regardless of electronegativity.
The formal charges of a neutral molecule sum to zero and those of an ion sum to the ion's charge.
The preferred structure has formal charges closest to zero and any negative charge on the most electronegative atom.
For H–C≡N all formal charges are zero, so it is preferred over the structure with carbon −1 and nitrogen +1.
BH4−
BH4− boron has 3 − 0 − 4
−1.
−1.
Section 6 of 9Limitations of Lewis structures
Limitations of Lewis structures
Odd-electron molecules are free radicals; they are highly reactive and few are stable.
Electron-deficient species such as BF3 have a sextet.
Expanded octets occur with a nonmetal central atom of period 3 or below surrounded by highly electronegative atoms.
- Phosphorus in PCl5 holds 10 electrons and sulfur in SF6 holds 12.
- The most common expanded-octet atoms are S and P.
Section 7 of 9More from the specification
More from the specification
Valence shell
- The outermost occupied electron shell of an atom containing the electrons directly involved in chemical bonding and valence sharing.
Octet rule
- The principle that main-group atoms tend to bond so that each atom possesses eight electrons in its valence shell, mimicking noble gas configurations.
Formal charge
- The apparent electronic charge assigned to an atom in a Lewis structure, calculated as: Valence electrons - Nonbonding electrons - 1/2(Bonding electrons).
Resonance structures
- Two or more valid Lewis formulas for a single molecule that cannot be represented accurately by one single structure alone.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Lewis diagram | Diagram of valence electrons as bonding and lone pairs. |
| Octet rule | Atoms are most stable with eight valence electrons. |
| Resonance hybrid | Actual structure, a mixture of resonance structures. |
| Formal charge | V − N − B/2 for an atom in a structure. |
| Expanded octet | More than eight electrons around a period 3 or lower atom. |
| Free radical | Species with an unpaired electron. |
Section 9 of 9Exam technique
1
Forgetting to add or remove electrons for the charge of an ion
2
Letting hydrogen have more than two electrons
3
Assuming resonance structures flip back and forth — the real structure is one hybrid
4
Applying the expanded octet to period 2 elements such as N or O
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