Lewis Structure Helper
Count valence electrons, adjust for charge, choose a central atom, and compare common Lewis structure templates.
What this helper gives you
This tool is a Lewis structure starting guide. It counts valence electrons, estimates a central atom, shows a simple structure sketch, and flags common resonance or shape hints for frequently used molecules and polyatomic ions.
| Output | How to use it |
|---|---|
| Total valence electrons | Start every Lewis dot structure with this electron budget |
| Charge adjustment | Add electrons for negative charge; subtract electrons for positive charge |
| Central atom hint | Usually the less electronegative non-hydrogen atom |
| Template note | Use it for common classroom molecules such as H2O, CO2, NH3, NO3-, and CO3^2- |
| Resonance note | Check whether equivalent structures need to be drawn |
Lewis structure steps
- Count valence electrons from every atom.
- Add electrons for a negative ion or subtract electrons for a positive ion.
- Choose the central atom, usually the least electronegative atom except hydrogen.
- Connect atoms with single bonds first.
- Complete outside atom octets, then place remaining electrons on the central atom.
- Use double or triple bonds if the central atom still lacks an octet.
- Check formal charges and resonance structures.
Formula used
Worked examples
H2O: oxygen has 6 valence electrons and two hydrogens add 2 more, so total = 8. Two O-H bonds use 4 electrons, leaving 4 electrons as two lone pairs on oxygen.
CO2: carbon contributes 4 and two oxygens contribute 12, so total = 16. The common structure is O=C=O with two lone pairs on each oxygen.
NO3-: nitrogen contributes 5, three oxygens contribute 18, and the -1 charge adds 1 electron, so total = 24. Nitrate has three equivalent resonance forms.
Common template data
| Species | Valence electrons | Shape hint | Resonance |
|---|---|---|---|
| H2O | 8 | Bent | No |
| CO2 | 16 | Linear | No major equivalent forms |
| NH3 | 8 | Trigonal pyramidal | No |
| CH4 | 8 | Tetrahedral | No |
| NO3- | 24 | Trigonal planar | Yes |
| CO3^2- | 24 | Trigonal planar | Yes |
Important limitations
Lewis structures are models. This helper does not replace formal charge checking, resonance drawing, VSEPR reasoning, or molecular orbital ideas for advanced molecules. It is best for common main-group classroom structures and quick electron-budget checks.
Common mistakes
- Forgetting to add electrons for negative ions.
- Putting hydrogen in the center of a molecule.
- Completing the central atom before outside atoms in simple structures.
- Ignoring formal charges when more than one structure seems possible.
- Drawing only one resonance form for nitrate, carbonate, or similar ions.
Related Chemistry Tools
Lewis Structure Helper FAQs
Which pair of elements has the most similar Lewis structures?
Pairs of elements in the same group of the periodic table have the most similar Lewis dot structures because they have the same number of valence electrons. Oxygen and sulfur both have 6 valence electrons; nitrogen and phosphorus have 5; carbon and silicon have 4; fluorine and chlorine have 7. The dot pattern is similar even though the element symbol changes.
How to draw the Lewis structure?
Use five steps: count total valence electrons, choose the central atom, connect atoms with single bonds, place remaining electrons as lone pairs on outer atoms first, then form multiple bonds if the central atom still lacks an octet. Always recount the electrons at the end.
What is an electron dot diagram?
An electron dot diagram, or Lewis dot symbol, represents valence electrons as dots around an element symbol. Single dots are placed before pairing, and molecule diagrams show shared pairs as bonds and unshared pairs as lone-pair dots.
How do you go from molecules to moles?
Use Avogadro's number: 1 mole = 6.022 x 10^23 molecules. To convert molecules to moles, divide by 6.022 x 10^23. To convert moles to molecules, multiply by 6.022 x 10^23. The Lewis drawing identifies one molecule; Avogadro's number counts how many such molecules are in a mole.
How to draw Lewis diagrams (worked steps)?
For H2O, valence electrons = 2(1) + 6 = 8. Put oxygen in the center, connect two hydrogens with single bonds, then place the remaining 4 electrons on oxygen as two lone pairs. Hydrogen gets a duet and oxygen gets an octet.
What is the Lewis structure for CO2?
For CO2, total valence electrons = 4 + 2(6) = 16. Carbon is central, and each oxygen forms a double bond to carbon. Each oxygen keeps two lone pairs, carbon has an octet through two double bonds, and the molecule is linear.
What is the Lewis structure for H2O?
In water, oxygen is central. Two H-O single bonds use 4 electrons, and the remaining 4 electrons form two lone pairs on oxygen. The electron geometry gives a bent molecule with a bond angle of about 104.5 degrees.
Which is the Lewis structure for H3PO4?
In phosphoric acid, phosphorus is central. A common classroom structure is P bonded to four oxygens: one P=O and three P-O-H groups. Each oxygen has lone pairs, and the three OH groups explain the acidic hydrogens.
What is the Lewis dot structure for HCN?
For hydrogen cyanide, total valence electrons = 1 + 4 + 5 = 10. Carbon is central, H-C is a single bond, C and N share a triple bond, and nitrogen keeps one lone pair. The molecule is linear.
What is the Lewis dot structure for NH3?
For ammonia, total valence electrons = 5 + 3(1) = 8. Nitrogen is central, forms three N-H single bonds, and keeps one lone pair. The lone pair gives ammonia a trigonal pyramidal shape and makes NH3 a Lewis base.
What is the Lewis structure for SO2?
For sulfur dioxide, total valence electrons = 6 + 2(6) = 18. A useful classroom drawing shows resonance between O=S-O and O-S=O forms. The real molecule has two equivalent S-O bonds of intermediate length and a bent shape.