Chemistry · Unit 2: Molecular and Ionic Compound Structure and Properties · 14 min read · Updated 2026-05-11
Lewis diagrams — AP Chemistry
AP Chemistry · Unit 2: Molecular and Ionic Compound Structure and Properties · 14 min read
1. Lewis Diagram Basics: Step-by-Step Construction★★☆☆☆⏱ 5 min
Lewis diagrams (also called Lewis dot structures) are 2D representations of covalent molecules and polyatomic ionic compounds that show how atoms are bonded together and where all valence electrons are located. Standard notation uses element symbols for the atomic nucleus and core electrons, single lines for single bonds (2 shared electrons), double lines for double bonds (4 shared electrons), triple lines for triple bonds (6 shared electrons), and dots for non-bonding lone pairs.
Determine atomic connectivity: the least electronegative atom is almost always the central atom
Calculate total valence electrons: add valence electrons for all atoms, add 1 per negative charge, subtract 1 per positive charge
Draw one single bond between each connected atom pair, subtract bonding electrons from total to get remaining non-bonding electrons
Distribute remaining electrons as lone pairs starting with terminal atoms to satisfy the octet guideline, then assign leftover to the central atom
If the central atom lacks a full octet, convert terminal lone pairs into multiple bonds to complete the octet
Exam tip: When adjusting for charge, explicitly write 'add electrons for negative charge, subtract for positive' next to your work to avoid flipping the rule under exam pressure.
2. Formal Charge and Preferred Lewis Structures★★★☆☆⏱ 4 min
Many molecules and ions can be drawn with multiple valid Lewis structures that all satisfy the octet guideline. To identify the most stable (preferred) structure, we use formal charge.
The formula for formal charge is:
FC = V - N - \frac{B}{2}
Where $V$ = number of valence electrons in the neutral free atom, $N$ = number of non-bonding electrons on the atom, $B$ = total number of bonding electrons shared by the atom. Three rules determine the preferred structure:
1. The most stable structure has the fewest atoms with non-zero formal charge
2. Any negative formal charge should be located on the most electronegative atom
3. Adjacent atoms should not have formal charges of the same sign
Exam tip: Always check that the sum of all formal charges equals the net charge of the species to catch arithmetic errors before selecting your answer.
3. Octet Rule Exceptions★★★☆☆⏱ 3 min
The octet rule is a general guideline derived from the stability of full valence s and p orbitals, not a physical law. Three common classes of exceptions are regularly tested on the AP exam:
**Electron deficient species**: Central atoms have fewer than 8 valence electrons, almost always for group 13 elements (boron, aluminum) with only 3 valence electrons to share
**Odd-electron species (free radicals)**: Total valence electrons is odd, so at least one atom has an unpaired electron and only 7 valence electrons. Common examples: $\text{NO}$, $\text{NO}_2$
**Expanded octet (hypervalent) species**: Central atoms have more than 8 valence electrons, only possible for period 3 or lower central atoms (have empty d-orbitals to accommodate extra electrons). Period 2 elements can never have expanded octets.
Exam tip: Any AP multiple-choice option showing an expanded octet on a period 2 central atom is automatically incorrect.
4. AP-Style Concept Check★★★★☆⏱ 2 min
Common Pitfalls
Why: Students confuse anion and cation charge adjustment, memorizing 'add for charge' without checking the sign
Why: Students reverse the connectivity rule, assuming more electronegative atoms attract more electrons so they belong in the center
Why: Students add extra electrons to get more favorable formal charges, forgetting the orbital restriction for period 2 elements
Why: Students focus on getting the electron arrangement right and skip notation requirements that cost FRQ points
Why: Students confuse total molecule electron count with per-atom octet count
Why: Students stop after counting non-zero formal charges and forget the second rule for preferred structures