Repair covalent structures with two electron checks

Count the whole molecule and each atom differently

A water drawing with just H–O–H has two bond lines, representing four distinct electrons. Neutral water supplies eight valence electrons: oxygen contributes six and each hydrogen contributes one. The drawing is therefore missing four electrons even though both hydrogen atoms already have a shared pair.

Try each supplied water proposal. One omits electrons, one assigns them consistently, and another uses the correct total in a way that gives hydrogen too many. Predict the failure before inspecting the ledger. A correct total and correct local assignments are separate requirements.

Shared pairs do not multiply electrons

A covalent bond involves electron density shared between atomic nuclei. In a Lewis structure, a single line represents one shared pair of electrons. A double bond has two shared pairs; a triple bond has three. A lone pair is a pair of nonbonding electrons assigned to one atom in the drawing. These lines and pair labels are representations, not rigid sticks and stationary dots observed in a photograph.

For a whole-molecule budget, count each bond pair once. For a local shell check, include the shared pair at each of the two bonded atoms. Those local tallies overlap. Adding them together would count bonding electrons twice and is not a way to compute the total number of electrons.

Hydrogen’s local target is two electrons, often called a duet. For the selected C, N and O examples, the useful local target is eight, an octet. In the accepted water structure, oxygen has four electrons in two lone pairs plus four electrons shared in two bonds. Each hydrogen shares two. The total is still eight, not eight plus two plus two.

Work systematically rather than decorating a sketch

Start by adding the valence contributions of every atom. For an ion, add electrons for a negative net charge or subtract them for a positive net charge; the current cases are neutral. Establish connectivity, place bond pairs, distribute remaining electrons and then audit both the total and each local count. Do not invent extra electrons to make a troublesome atom look complete.

Water has 6 + 1 + 1 = 8 available electrons. Two single bonds use four, leaving four for two lone pairs on oxygen. Ammonia has 5 + 3 = 8: three single N–H bonds use six, leaving one nitrogen lone pair. Both examples complete hydrogen duets and the central-atom octet without changing the total budget.

Carbon dioxide has 4 + 6 + 6 = 16 available electrons. A proposal with two single C–O bonds and three lone pairs on each oxygen uses all sixteen, but leaves carbon with only four local electrons. Moving a lone pair from each oxygen into an additional shared bond yields O=C=O with two lone pairs on each oxygen. The total remains sixteen while each selected atom reaches an octet. This describes a change in representation, not a time-resolved reaction animation.

Nitrogen N2 supplies ten valence electrons. A triple bond uses six; one lone pair on each nitrogen supplies the remaining four. Each nitrogen locally counts six shared electrons and its two lone-pair electrons. Compare that with the supplied single-bond proposal: its whole-molecule budget can match while both nitrogen local counts remain incomplete.

Formal charge is another ledger, not a measured charge map

Formal charge divides every bond’s electrons equally between its two atoms. Subtract the assigned electrons from the neutral atom’s valence count:

Formal charge = valence electrons − nonbonding electrons − half the bonding electrons.

For oxygen in the accepted water drawing, this gives 6 − 4 − 2 = 0. In the all-single-bond CO2 proposal, carbon has formal charge +2 and each oxygen −1. The sum is zero, but that alone does not resolve carbon’s incomplete octet. A neutral overall charge is not a substitute for either earlier check.

Formal charge differs from partial charge, which describes unequal electronic distribution in a bond. Equal formal charges do not require equal attraction for shared electrons. Later resonance work may compare multiple Lewis representations of the same species; those are not molecules physically switching between drawings.

Transfer: methane has four H atoms bonded to one C atom. Predict its total valence electrons, the number of single bonds, and whether carbon needs any lone pairs in the conventional Lewis representation.

Check the methane ledger

Carbon contributes four and the hydrogens contribute four, giving eight electrons. Four single C–H bonds use all eight. Each H has a duet and carbon counts eight shared electrons locally. Carbon has no lone pairs in this representation. Summing the five atoms' local counts would incorrectly count the bond electrons twice.

The checker deliberately handles only four supplied molecules and three proposals each. Passing its tests does not determine bond lengths, energies, three-dimensional geometry or stability. Electron-deficient compounds, odd-electron species, resonance and expanded valence descriptions require additional reasoning. The octet guideline is useful within a stated scope, not a universal law.

OpenStax Lewis structures provides reference explanations. Continue to molecular shape and polarity to distinguish a connectivity drawing from geometry.