Use this method whenever you need to generate a Lewis structure by hand. It works for many neutral molecules and polyatomic ions, while the exception sections explain where resonance, incomplete octets and expanded valence shells require extra care.
A Lewis structure accounts for valence electrons, not every core electron. Lines represent shared electron pairs and dots represent nonbonding electrons. The final drawing should use the correct total electron count and make formal charges as reasonable as possible.
Fast checklist: electrons → skeleton → single bonds → terminal octets → central atom → multiple bonds → formal charges → resonance.
The six-step method
Count total valence electrons.
Add the group valence of every atom. Add one electron for each negative charge and subtract one for each positive charge.
Choose the atom arrangement.
The least electronegative suitable atom is often central. Hydrogen is never central, and halogens are usually terminal.
Connect atoms with single bonds.
Each line uses two electrons. Subtract the bonding electrons from the total.
Complete terminal octets.
Give terminal atoms lone pairs until they reach an octet. Hydrogen needs only two electrons.
Complete the central atom.
Place remaining electrons on the central atom. If it lacks an octet, convert adjacent lone pairs into double or triple bonds when chemically reasonable.
Verify charges and resonance.
Calculate formal charges, check the total charge and draw equivalent contributors when electrons can be placed in more than one valid way.
Example: CO2
CO2 has 16 valence electrons. Begin with O–C–O, complete oxygen octets, then form two C=O double bonds so carbon reaches an octet and all formal charges become zero.
Include the charge when counting electrons, enclose the completed Lewis structure in brackets and place the overall charge outside. For example, NH4+ has one fewer electron than the neutral atom total, while NO3− has one additional electron.