How to Draw Lewis Structures Step by Step
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Drawing Lewis structures becomes easier when you treat the diagram as an electron-counting puzzle. Count the available valence electrons, build a sensible skeleton, add bonds and lone pairs, then check that the final structure uses the correct total.
Start with the electron count
Start by counting valence electrons for every atom in the formula. Main-group elements in the same periodic-table group often have the same number of valence electrons. Add the totals together. For an ion, add one electron for each negative charge or subtract one for each positive charge.
Next choose a central atom and connect surrounding atoms with single bonds. Hydrogen is never the central atom because it forms only one bond in ordinary Lewis structures. The least electronegative suitable atom is often central, although there are exceptions. Each single bond represents two shared electrons, so subtract those bonding electrons from your total.
Place the remaining electrons as lone pairs around the outer atoms first, usually aiming to give them full outer shells. Hydrogen needs only two electrons. Many common main-group atoms are most stable with eight electrons around them, which is the octet rule. This is the basic routine for drawing Lewis structures.
Multiple bonds can solve an incomplete octet
Sometimes you run out of electrons before the central atom reaches an octet. In that case, a lone pair from a neighbouring atom can sometimes be turned into an additional shared pair, creating a double or triple bond. Carbon dioxide is a familiar example: O=C=O gives carbon and both oxygens complete octets with the available electrons.
When drawing Lewis structures, never invent extra electrons just to make the picture look tidy. The total number of dots and bonding electrons must match the valence-electron count you found at the start. That count is one of your strongest error checks.
The octet rule is useful but not universal. Boron can appear in stable compounds with fewer than eight electrons, while elements in the third period and below can sometimes be shown with expanded valence shells in common Lewis models. Odd-electron species also exist. Learn the basic octet method first, then treat exceptions as a separate layer.
Formal charge helps compare possible structures
More than one Lewis structure may satisfy the basic electron count. Formal charge can help compare them. For a particular atom, formal charge is its valence-electron number minus its nonbonding electrons minus half of its bonding electrons. The formal charges across a neutral molecule should add to zero, while those in an ion should add to the ion's overall charge.
Structures with smaller formal charges are often preferred, and negative formal charge is generally more reasonable on more electronegative atoms. These are guidelines rather than a substitute for real bonding theory, but they make Lewis diagrams more useful for introductory chemistry.
A final routine for drawing Lewis structures is simple: count electrons, draw the skeleton, complete outer atoms, place leftovers on the centre, consider multiple bonds, and check formal charges. Work slowly enough that every dot has a reason to be there.
The takeaway
Drawing Lewis structures is mostly careful bookkeeping. Count valence electrons first, use two electrons per bond, add lone pairs, check octets where the rule applies, and make sure the total electron count never changes. If several structures work, formal charge can help you judge which representation is more reasonable.