Calculator Inputs
Formula Used
Formal charge (FC) estimates how charge is distributed on an atom within a Lewis structure:
FC = V − (N + B/2)
- V = valence electrons for the neutral atom.
- N = nonbonding electrons (lone-pair electrons on the atom).
- B = bonding electrons (shared electrons in bonds around the atom).
- B/2 assigns half of shared electrons to the atom.
How to Use This Calculator
- Identify the atom in your Lewis structure and its valence electrons.
- Count lone-pair electrons on that atom to get N.
- Enter bonding electrons B, or use bonds × electrons per bond.
- Press calculate to view the formal charge and the step-by-step breakdown.
- Use the download buttons to save a CSV or PDF summary.
Example Data Table
| Atom (in molecule) | V | N | B | FC = V − (N + B/2) |
|---|---|---|---|---|
| O in H₂O | 6 | 4 | 4 | 0 |
| N in NH₄⁺ | 5 | 0 | 8 | +1 |
| O (single-bonded) in NO₃⁻ resonance form | 6 | 6 | 2 | −1 |
| O (double-bonded) in NO₃⁻ resonance form | 6 | 4 | 4 | 0 |
Notes for Accurate Results
- Formal charge depends on the Lewis structure you choose.
- Resonance structures redistribute B and N across atoms.
- For expanded octets (e.g., third-row elements), verify bonding electron counts carefully.
- Use the same electron-counting convention across your whole structure.
Professional Article
What Formal Charge Represents
Formal charge is a bookkeeping tool for Lewis structures, not a measured atomic charge. It assumes electrons in bonds are shared equally and helps compare plausible structures. Lower-magnitude charges and negative charge on more electronegative atoms usually indicate a better representation of electron distribution in molecules and polyatomic ions.
Core Electron-Counting Inputs
The calculator uses valence electrons V for the neutral atom, nonbonding electrons N from lone pairs, and bonding electrons B from shared pairs. For many main‑group atoms, V matches periodic-table group trends. N is counted directly from the drawn structure, while B totals electrons participating in bonds around the atom.
Using Bonds When Bonding Electrons Are Unknown
If you do not know B directly, you can enter the number of bonds and electrons per bond. A single bond contributes about two bonding electrons, a double bond four, and a triple bond six. Summing these around the atom gives B, which is then halved in the formal-charge formula.
Typical Formal-Charge Patterns in Common Ions
Common reference points help validate inputs. In water, oxygen has V=6, N=4, B=4, giving FC=0. In ammonium, nitrogen has V=5, N=0, B=8, giving FC=+1. In nitrate resonance forms, singly bonded oxygen often gives −1, while the double-bonded oxygen gives 0.
Resonance and Charge Delocalization
Resonance does not change the total charge of the ion, but it redistributes formal charge across equivalent atoms. For example, NO3− can place the negative charge on any oxygen in different resonance structures. Use the calculator atom-by-atom for each resonance form to see how formal charge shifts among positions.
Octet Expansion and Hypervalent Cases
Third-row and heavier elements may appear hypervalent in Lewis drawings, affecting B and N counts. For species like SF6 or XeF4, formal charge can still be computed, but the bonding model is simplified. Carefully count bonding electrons per bond type, and confirm the overall structure matches the known molecular formula and charge.
Interpreting Results for Structure Selection
When multiple Lewis structures satisfy octets, prefer arrangements that minimize large formal charges and avoid placing positive charge on highly electronegative atoms when reasonable alternatives exist. Formal charge is one criterion alongside resonance stability and electronegativity. Use the step-by-step output to identify which input change drives the charge difference.
Practical Checks for Labs and Coursework
After computing each atom’s formal charge, sum all formal charges and verify the total equals the molecule or ion’s overall charge. If the sum is wrong, revisit B and N counts. For ions, ensure brackets and charge notation are consistent. Export the CSV or PDF to document assumptions and show your working clearly.
FAQs
What values should I enter for valence electrons (V)?
Use the neutral atom’s valence count, typically matching its periodic-group trend. For main‑group atoms, this is the electrons outside closed shells, not the total electrons.
How do I count nonbonding electrons (N)?
Count electrons in lone pairs on the atom in your Lewis structure. Each lone pair contributes two electrons, so two lone pairs equal N=4.
What is bonding electrons (B) in this calculator?
B is the total shared electrons in all bonds around the atom. Two single bonds give B=4, one double bond gives B=4, and one triple bond gives B=6.
Why do I sometimes get a non-integer formal charge?
Formal charges are usually integers for a single Lewis structure. Non-integers often indicate mixed bond types entered as averages, inconsistent electron counting, or resonance being represented as a single blended structure.
How do I handle resonance structures?
Evaluate each resonance form separately. The overall ion charge stays constant, but formal charge shifts among equivalent atoms. Compare results to see which atoms carry negative or positive charge in each form.
Does formal charge equal real atomic charge?
No. Formal charge is a bookkeeping assignment based on equal sharing of bonding electrons. Real charge distributions come from electron density and are better described by methods like partial charges.
What is a good final check after calculating?
Sum formal charges for all atoms in the structure and confirm the total equals the molecule’s stated charge. If not, recheck lone pairs, bond orders, and any derived bonding-electron inputs.
Accurate formal charges help predict bonding and reactivity better.