Drawing Lewis Structures Step by Step

Add the valence electrons of every atom, then adjust for charge: add one for each negative charge, subtract one for each positive charge. This number is a budget you must spend exactly, no more and no less.

Write the electron budget visibly and update the remaining balance after every operation. A single bond spends two electrons, a double bond four and a lone pair two. For an ion, the charge is applied to the sum once: NO₃⁻ has 5 + 3(6) + 1 = 24, not 24 plus an extra electron on each oxygen. Core electrons do not appear in this valence-electron drawing.

The central atom is normally the least electronegative atom that is not hydrogen. Carbon is central in CO₂ and CH₄, sulfur in SO₂ and sulfate. Hydrogen and fluorine are always terminal because they can form only one bond.

The formula alone does not always establish connectivity. In CH₃OH, the carbon bonds to three H atoms and O, while O bonds to H; forcing every atom onto one center gives the wrong molecule. Use a supplied structural formula or known connectivity when available. Hydrogen is terminal in ordinary Lewis structures; fluorine is also normally terminal. For larger molecules, construct the skeleton first and complete each local center rather than insisting on one central atom.

Place the remaining electrons as lone pairs on the outer atoms until each has an octet, hydrogen excepted. If electrons remain, put them on the central atom only within its permitted valence pattern. Expanded-valence bookkeeping is available for suitable period-3 or heavier centers; an overfilled period-2 center means the drawing must be reconsidered.

The last operation changes where electrons sit, not how many exist. The final O=C=O has eight bonding electrons and eight nonbonding electrons, two lone pairs on each O. Counting eight electrons around C plus eight around each O would count every shared pair twice.

If the central atom still lacks an octet, pull a lone pair from an outer atom into the space between the two atoms to make a double or triple bond. The total electron count never changes when you do this, the electrons simply move.

In HCN, ten electrons must serve three atoms. The final H–C≡N contains four shared pairs, eight electrons, plus one nitrogen lone pair, two electrons. Carbon has four bond lines and an octet; nitrogen has a triple bond and a lone pair, also an octet. Do not add a lone pair to carbon afterward just because another structure had one. Lone-pair placement follows the fixed budget and charge checks.

After placing bonds and lone pairs, recount all electrons against the original total, check each atom's allowed valence pattern, and sum formal charges to the species charge. These are separate tests: a drawing can have the correct electron total yet place a multiple bond on the wrong atom, or satisfy octets while representing the wrong ion. Use all three before accepting the structure.

The complete audit is: count every drawn electron once; inspect the duet, octet or justified exception at each atom; calculate formal charges and check their sum; then ask whether alternative valid electron placements require resonance. Lewis drawings show connectivity and electron bookkeeping, not a measured three-dimensional bond angle. Shape is determined after the structure is established.