Click to build Lewis structures for any molecule.
Drawing a correct Lewis structure is the foundation every subsequent structural concept — VSEPR geometry, hybridization, molecular polarity — builds on, which is why getting formal charge and octet rules right by hand is emphasized so heavily in AP Chemistry Unit 2 and tested constantly on IChO.
Formal charge, calculated as (valence electrons) - (lone pair electrons) - ½(bonding electrons), identifies which of several possible Lewis structures for the same molecular formula is actually the best representation. The preferred structure minimizes formal charges overall and places any necessary negative formal charge on the more electronegative atom — this single rule resolves most ambiguous cases students encounter.
The octet rule (eight electrons around each atom, achieved through bonding and lone pairs) holds reliably for period 2 elements, but period 3 and beyond elements like phosphorus, sulfur, and chlorine can expand beyond the octet using available d orbitals, forming structures like PCl5 (10 electrons around P) or SF6 (12 electrons around S) that would be impossible to draw correctly under a strict octet assumption.
Resonance structures arise when a single Lewis structure cannot adequately represent a molecule's actual bonding — the true structure is a weighted average (resonance hybrid) of all valid contributing structures, not an equilibrium flipping between them. Ozone (O3) and the carbonate ion (CO3²⁻) are classic examples: experimentally, all bonds in these species are equal length, intermediate between single and double bond lengths, exactly as the resonance hybrid concept predicts.
Radical species, with an odd total electron count, cannot satisfy the octet rule on every atom no matter how electrons are arranged — one atom is left with an unpaired electron, and identifying which atom should bear that radical (typically the least electronegative, most stable position) requires the same formal-charge reasoning used elsewhere.
This Lewis structure builder gives real-time formal charge and octet-rule feedback as you place bonds and lone pairs, handles expanded octets and radicals correctly, and helps you find the lowest-formal-charge structure among multiple resonance contributors.