Formal Charge Calculator for Lewis Structures
Calculate an atom’s formal charge from neutral-atom valence electrons, nonbonding electrons and total bond order. You can also solve one missing term or check whether every atom in a Lewis structure adds to the expected molecular or ionic charge.
- Single-atom inverse solver
- Complete structure charge check
- Main-group element presets
- Clear electron-counting steps
Online Formal Charge Calculator
Use one row for each distinct bonding and lone-electron environment in the Lewis structure. Count single, double and triple bonds as bond orders 1, 2 and 3.
The oxygen atom in this H2O Lewis structure has formal charge 0.
Atom-by-Atom Breakdown
| Atom | Qty | V | N | BO | FC each | Subtotal |
|---|---|---|---|---|---|---|
| O | 1 | 6 | 4 | 2 | 0 | 0 |
Calculation Steps
- Oxygen supplies 6 valence electrons as a neutral isolated atom.
- Two lone pairs contain 4 nonbonding electrons.
- Two single bonds give total bond order 2 and contain 4 bonding electrons.
- FC = 6 − 4 − 2 = 0.
Oxygen is assigned 4 nonbonding electrons plus one electron from each bond, for 6 assigned electrons. That equals the neutral atom’s valence count, so its formal charge is 0.
The result describes the Lewis structure entered. It is not a measured partial charge.
How to Use This Formal Charge Calculator
- Choose the mode. Select one atom for a direct or inverse calculation, or select the complete-structure check.
- Select an element or enter V. Use a main-group preset, or choose Custom and enter the neutral isolated atom’s valence electrons.
- Count nonbonding electrons. Enter every lone-pair electron and any unpaired nonbonding electron on that atom.
- Add the bond orders. Count each single bond as 1, each double bond as 2 and each triple bond as 3.
- Describe repeated atoms carefully. Combine atoms in one structure row only when their electron environments are equivalent.
- Enter the expected net charge. For a full structure, use 0 for a neutral molecule or the signed charge of the ion.
- Review the result. Check each atom’s formal charge, the subtotal, the net sum and the displayed calculation steps.
Formal Charge Formula
Formal charge assigns every nonbonding electron to its atom and divides bonding electrons equally between the bonded atoms. It compares that assigned count with the number of valence electrons in the neutral isolated atom.
For ordinary Lewis bonds: FC = V − N − BO
FC is formal charge, V is the neutral atom’s valence-electron count, N is the number of nonbonding electrons on the atom, B is the number of bonding electrons touching it, and BO is the sum of its bond orders. A single bond contains 2 bonding electrons, a double bond 4 and a triple bond 6. Therefore B/2 and BO give the same term for ordinary two-electron bonds.
The assigned-electron count is N + BO. If an atom is assigned fewer electrons than its neutral valence count, its formal charge is positive. If it is assigned more, its formal charge is negative. This is a bookkeeping convention for a stated Lewis structure. It does not claim that bonds are physically nonpolar or that charge is localized exactly as drawn.
Worked Formal Charge Examples
Oxygen in water
The oxygen in H2O has 6 neutral-atom valence electrons, 4 nonbonding electrons and two single bonds. Its total bond order is 2.
Nitrogen in ammonium
The nitrogen in NH4+ has four single bonds and no lone pair in the usual Lewis structure. It has V = 5, N = 0 and BO = 4.
Carbon monoxide
In the common :C≡O: Lewis structure, both atoms have one lone pair and a triple bond. Carbon gives 4 − 2 − 3 = −1. Oxygen gives 6 − 2 − 3 = +1. The two formal charges sum to 0, matching neutral CO.
One nitrate resonance contributor
For one conventional NO3− contributor, central N has four total bond orders and no nonbonding electrons, so it is +1. The double-bonded O is 0. Each of the two single-bonded O atoms is −1. The sum is +1 + 0 − 1 − 1 = −1.
Neutral-Atom Valence Electron Guide
For the main-group presets, the calculator uses the ordinary outer-shell counts taught with Lewis structures. Groups 1 and 2 normally contribute 1 and 2 valence electrons. Groups 13 through 18 contribute 3 through 8, except helium, which has 2.
| Periodic group | V used | Common examples | Important note |
|---|---|---|---|
| 1 | 1 | H, Li, Na, K | Hydrogen follows a duet rather than an octet. |
| 2 | 2 | Be, Mg, Ca | These elements are less common in simple covalent Lewis structures. |
| 13 | 3 | B, Al, Ga | Boron compounds may be electron deficient. |
| 14 | 4 | C, Si, Ge | Carbon commonly forms four total bond orders. |
| 15 | 5 | N, P, As | Heavier members may appear in expanded-valence drawings. |
| 16 | 6 | O, S, Se | Oxygen commonly has two total bond orders in neutral structures. |
| 17 | 7 | F, Cl, Br, I | A singly bonded halogen with three lone pairs has FC = 0. |
| 18 | 8 | Ne, Ar, Kr, Xe | Helium is the exception and uses V = 2. |
Transition-metal and inner-transition electron counting needs a stated bonding model and often differs from elementary main-group rules. The presets intentionally omit those elements. Choose Custom only when your course, reference or validated model supplies the V value to use.
Checking a Complete Lewis Structure
The formal charges on all atoms must add to the overall charge of the represented molecule or ion. A neutral molecule must sum to 0. A polyatomic ion must sum to its signed ionic charge. The structure mode multiplies each atom-type charge by its quantity before adding the subtotals.
A matching sum is a necessary consistency check, but it does not prove that the Lewis structure is the best contributor or even chemically reasonable. You must still check the total valence-electron count, connectivity, octets or applicable exceptions, charge separation and electronegativity.
When several Lewis structures have the same atomic arrangement but different electron placement, calculate each resonance contributor separately. Do not enter an averaged bond order such as 1.5 into this tool. Formal charges are attached to one explicit Lewis contributor. The real resonance hybrid is not switching between drawings and its electron distribution is not fully described by one contributor.
Formal Charge Versus Oxidation State and Partial Charge
| Quantity | Electron assignment rule | Typical use | Main caution |
|---|---|---|---|
| Formal charge | Split every covalent bond equally | Compare Lewis structures and track electron movement | A bookkeeping value, not a measured atomic charge |
| Oxidation state | Use an ionic approximation for heteronuclear bonds | Redox accounting and nomenclature | May differ sharply from formal charge |
| Partial charge | Derived from a model or experimental interpretation of electron density | Polarity, interactions and computational chemistry | Depends on the method and is often nonintegral |
For water, each atom has formal charge 0 in the usual Lewis structure, yet oxygen is partially negative and hydrogen partially positive because the O–H bonds are polar. For carbon monoxide, the common Lewis contributor assigns C −1 and O +1, while other charge descriptions depend on the property and model being discussed.
Radicals, Coordinate Bonds and Expanded Valence
An unpaired electron drawn on one atom counts as one nonbonding electron. The calculator accepts odd N values for radical Lewis structures. It still applies the same formal charge formula. It does not decide where the radical electron should be placed or whether a proposed radical structure is stable.
After a coordinate covalent bond forms, formal-charge bookkeeping splits that bond equally, just like any other ordinary two-electron bond. Count it as bond order 1. Do not assign both bonding electrons back to the donor when calculating formal charge.
For a heavier main-group atom in an expanded-valence Lewis drawing, enter every explicit bond order and nonbonding electron. The arithmetic remains valid for the drawing. The calculator does not judge whether an expanded-octet, charge-separated or alternative resonance description is the most useful modern bonding model.
Common Formal Charge Mistakes
- Entering the ion’s electron count instead of the neutral isolated atom’s V value.
- Entering lone pairs rather than nonbonding electrons. Three lone pairs mean N = 6.
- Counting a double bond as one connection instead of bond order 2.
- Counting all bonding electrons instead of one-half of them.
- Combining repeated atoms that are not equivalent in the chosen resonance contributor.
- Expecting a molecular formula alone to determine bond placement and lone pairs.
- Assuming a correct net-charge sum proves that the Lewis structure is preferred.
- Confusing formal charge with oxidation state, ionic charge or computed partial charge.
- Averaging resonance bond orders before assigning contributor-specific formal charges.
- Using a main-group group-number shortcut for a transition-metal complex without a stated model.
Validation, Privacy and Calculator Limits
The calculator accepts signed whole numbers only. It limits V to 1 through 18, N to 0 through 32, total bond order to 0 through 16, and one row’s formal charge to the range generated by those inputs. Structure mode supports up to 20 distinct atom types and up to 1,000,000 equivalent atoms per row.
All calculations stay in your browser. No formula, structure or result is sent to a server or stored as calculation history. Changing an input clears the displayed result so an earlier calculation cannot be mistaken for the current data.
This is an electron-bookkeeping tool, not a Lewis-structure generator, quantum-chemistry calculation, valence-bond analysis or molecular-orbital solver. Verify consequential chemical, laboratory, pharmaceutical, environmental and safety work against an approved reference and qualified review.
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Formal Charge Calculator FAQs
What is the formula for formal charge?
Formal charge is FC = V − N − B/2, where V is the neutral atom’s valence electrons, N is its nonbonding electrons and B is its bonding electrons. For ordinary Lewis bonds, B/2 equals total bond order.
How do I count bonding electrons?
A single bond contains 2 bonding electrons, a double bond 4 and a triple bond 6. You may enter half of this total directly as bond order: 1, 2 or 3 per bond.
Are lone pairs entered as pairs or electrons?
Enter electrons. One lone pair is 2 nonbonding electrons, two lone pairs are 4 and three lone pairs are 6. An unpaired nonbonding radical electron counts as 1.
What should the formal charges in a molecule add up to?
They must add to the overall charge of the represented structure. A neutral molecule sums to 0. A singly charged anion sums to −1, and a singly charged cation sums to +1.
Is formal charge the same as oxidation state?
No. Formal charge splits bonding electrons equally. Oxidation state uses an ionic approximation for heteronuclear bonds. The two values answer different bookkeeping questions.
Is formal charge the actual charge on an atom?
No. It is a Lewis-structure bookkeeping value. Actual electron density is not generally divided equally, and a calculated or interpreted partial charge depends on the method used.
How do resonance structures affect formal charge?
Calculate each explicit resonance contributor separately because bond and lone-pair placement changes. Do not average the bond orders before assigning contributor-specific formal charges.
Can this calculator handle radicals?
Yes. Enter an unpaired electron as one nonbonding electron. The tool calculates the formal charge of the entered drawing but does not choose the preferred radical site.
Does a coordinate bond count differently?
No. Once the two-electron bond is formed, formal-charge bookkeeping divides it equally. Count a coordinate single bond as bond order 1.
Can a correct net formal charge prove my Lewis structure is right?
No. A matching sum is necessary but not sufficient. Also check total valence electrons, connectivity, octets or valid exceptions, charge separation, electronegativity and resonance.
Method References
- IUPAC Gold Book, formal charge, the formal electron-assignment definition and equal-sharing convention.
- OpenStax Chemistry 2e, Formal Charges and Resonance, the formula, whole-structure sum check and Lewis-structure selection guidance.
- OpenStax Organic Chemistry, Formal Charges, electron ownership and common atom examples.
- OpenStax Chemistry 2e, Electron Configurations, main-group and transition-element valence-electron distinctions.
- IUPAC Gold Book, Lewis formula, formal-charge notation in complete valence-electron formulas.
- IUPAC Gold Book, oxidation state, the electron-assignment distinction between oxidation state and formal charge.
Disclaimer: This calculator provides educational formal-charge bookkeeping for the Lewis structure you enter. It does not generate or validate a complete molecular structure, measure atomic charge or replace approved chemical methods. Independently verify laboratory, pharmaceutical, environmental, regulatory, hazardous-material and safety-critical work.