# Lewis Structure & VSEPR Predictor

Predict the Lewis dot structure, valence electrons, central atom, lone pairs, and VSEPR geometry (linear / trigonal / tetrahedral / …) of a covalent molecule.

> Canonical page: https://elysiatools.com/en/tools/lewis-structure-and-vsepr-predictor

- **Category:** Utilities

- **Keywords:** lewis structure, vsepr, molecular geometry, bond angle, lone pair, valence electrons, octet rule, formal charge, axe notation

## Overview

Predict the Lewis dot structure and VSEPR molecular geometry of a covalent molecule, step by step.

**What you give it.** A molecular formula like `H2O`, `CO2`, `NH3`, `CH4`, `SF6`, `PCl5`, `XeF4`, or `SO2`. You can also specify the central atom (the tool will pick the least electronegative one if you don't) and the overall charge (for polyatomic ions like `SO4^2-` or `NH4+`).

**How it works.**
1. **Valence electron count.** Sum the valence electrons of every atom (from the periodic table), then add one electron for each negative charge (or subtract for positive).
2. **Central atom.** Usually the least electronegative atom (excluding H, which is always terminal). For example, in `SO2` sulfur is central, not oxygen.
3. **Skeleton.** Place the central atom in the middle, terminal atoms around it, connected by single bonds.
4. **Octet completion.** Distribute the remaining electrons to terminal atoms first (to complete their octets — 2 for H, 8 for everything else), then any leftover to the central atom.
5. **Multiple bonds.** If the central atom lacks an octet, convert terminal lone pairs into double or triple bonds (typically C, N, O, S).
6. **Formal charges.** Compute for every atom; if a lower-charge arrangement exists, suggest it.
7. **VSEPR.** Count bonding regions and lone pairs on the central atom → look up the `AX_nE_m` type → report the electron-domain geometry, the molecular shape, and the ideal bond angle.

**Geometry table covered.** Linear (AX2), Trigonal planar (AX3), Tetrahedral (AX4), Trigonal bipyramidal (AX5), Octahedral (AX6), plus the lone-pair variants: Bent (AX2E / AX2E2), Trigonal pyramidal (AX3E), Seesaw (AX4E), T-shape (AX3E2), Square pyramidal (AX5E), Square planar (AX4E2).

**Teaching notes.** Each result shows the full reasoning: how many valence electrons, which atom is central and why, how many lone pairs end up where, and the VSEPR type. This is the part students actually need.

**Scope & limits.** This tool handles main-group covalent molecules and common ions with a single central atom — the standard first-year chemistry curriculum. It does **not** handle hypervalent oddities, radicals, transition-metal complexes, resonance beyond a note, or multi-center bonding (e.g. boranes). For those, a chemistry textbook or a quantum-chemistry tool is the right answer.

## Inputs

- **Molecular formula** (text): e.g. H2O, CO2, NH3, SF6, XeF4
- **Central atom (optional)** (text): e.g. S (defaults to least electronegative non-H)
- **Charge** (select)

## When to use

- When verifying Lewis dot structures and formal charges for chemistry assignments.
- When determining the VSEPR molecular geometry, AXE notation, and ideal bond angles of a covalent compound.
- When teaching molecular geometry and needing step-by-step derivations of electron-domain shapes.

## How it works

- Enter the molecular formula and optionally specify the central atom and overall charge.
- The tool calculates the total valence electrons, determines the central atom, and distributes electrons to satisfy the octet rule.
- It computes formal charges and applies VSEPR theory to output the molecular shape, AXE classification, and ideal bond angles.

## Use cases

- Verifying homework answers for molecular geometry and AXE notation in general chemistry courses.
- Generating step-by-step Lewis structure diagrams and VSEPR predictions for chemistry lesson plans.
- Determining the ideal bond angles and lone pair counts of common covalent compounds.

## Frequently asked questions

### Can this tool handle polyatomic ions?

Yes, you can input formulas like SO4 or NH4 and select the corresponding charge from the dropdown to calculate the correct structure.

### How does the tool select the central atom?

It automatically selects the least electronegative non-hydrogen atom, but you can manually override this by entering a specific central atom.

### What VSEPR geometries are covered?

It supports linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral geometries, including lone-pair variants like bent, seesaw, and square planar.

### Does it support transition metals or radicals?

No, the predictor is limited to main-group covalent molecules and ions with a single central atom.

### Does the tool show the step-by-step calculation?

Yes, it provides the full reasoning, including valence electron summation, octet completion steps, and formal charge calculations.

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