Read AX₂E₂.
A is the central atom, X counts atoms bonded to it, and E counts its lone pairs. In water, two bonds plus two lone pairs give four electron regions. The electron geometry is tetrahedral; the atom-only shape is bent.
CHEMISTRY / FROM SHAPE TO SUBSTANCE
See the molecule.
Understand the amount.
Turn a formula into a shape, then a measurable quantity. Explore ten 3D models, trace each element’s contribution to molar mass, and convert grams, moles and molecule counts with the steps shown.
3D geometry · Mass ↔ moles · Element breakdown · Local, free, no account
Start with a question
AX₂E₂ / 4 ELECTRON REGIONS
Water H₂O
Preparing your molecular model…
Ball sizes and equal bond lengths are illustrative. Gold marks the inspected bond pair.
Loading interactive model. The reference information below works without JavaScript.
Molecular shapeBent
Electron geometryTetrahedral
Central lone pairs2
Two central lone pairs count toward electron geometry, but not toward the molecular shape.
From formula to amount
M = 18.015 g/molMassgrams
Amountmoles
Moleculescalculated count
Loading amount calculation…
Counts are estimates from an assumed pure sample and nominal atomic weights. Nₐ is exact; your sample measurement and the molar mass are not.
Every bond angle in this model
| Bond pair | Position types | Angle |
|---|---|---|
| H1–O–H2 | — | ≈ 104.5° |
Element composition & molar-mass calculation
| Element | Atoms / molecule | Atomic weight | Contribution · g/mol | Mass % | In your sample · g |
|---|
Mass percentage is not atom percentage. Values are based on the formula, not a laboratory assay. Atomic weights describe normal terrestrial isotopic composition, not a chosen isotope.
A MODEL YOU CAN QUESTION
See the shape. Understand the assumption.
Why not 109.47°?
Choose water, then switch between reference and ideal geometry. The H–O–H model changes from 104.5° to 109.47°. Lone-pair repulsion is part of the explanation, but VSEPR is a qualitative model, not an exact angle calculator.
Three angles, one molecule.
Choose PF₅. The equatorial pairs are 120°, axial-to-equatorial pairs 90°, and the opposite axial pair 180°. Rotate the view: the displayed angle stays the same because the calculation uses 3D directions.
What the inspector calculates
θ = arccos[(a · b) ÷ (|a| |b|)]
Each bond is a direction from the central atom. The angle is independent of the camera and is shown to two decimals for inspection, not as an experimental precision claim. JSON includes all pairs and dimensionless unit directions so you can reproduce the calculation.
18.015 g of water.
Using H = 1.0080 and O = 15.999 gives M(H₂O) = 2 × 1.0080 + 15.999 = 18.015 g/mol.
n = m / M = 18.015 / 18.015 = 1 mol
N = n × Nₐ = 6.02214076 × 10²³ molecules
One water molecule has three atoms, so that ideal sample represents 1.806642228 × 10²⁴ atoms. These values assume a pure sample with the tabulated nominal molar mass.
250 mmol of carbon dioxide.
Choose CO₂, switch to Mass & moles, choose Moles → calculate mass, then enter 250 mmol.
M = 12.011 + 2 × 15.999 = 44.009 g/mol
m = 0.250 × 44.009 = 11.00225 g
The number of molecules is 1.50553519 × 10²³. Gas volume is not inferred: that would require temperature, pressure and an explicit gas model.
Take the assumptions with you.
The JSON contains current input and unit, molar mass contributions, calculated mass, moles, molecules and element amounts, plus the selected geometry. Invalid inputs clear the amount result and lock both export actions until corrected.
Isotope enrichment, purity, solutions, ions, reaction stoichiometry and propagated measurement uncertainty are outside this tool.
What this lab does not claim
- Ball sizes and equal displayed bond lengths are illustrative, not to scale. Space-fill uses schematic spheres, not physical atomic radii.
- Only lone pairs on the central atom are shown. Translucent lobes are teaching symbols, not electron-density surfaces.
- Reference mode uses rounded textbook water and ammonia angles. Other models use idealized VSEPR directions, including distorted real SF₄ and ClF₃.
- Molar masses use nominal abridged atomic weights, not isotope-specific exact masses. Purity, uncertainty and reaction calculations are not included.
- No energy minimization, quantum calculation, arbitrary formula prediction or chemical-handling guidance is provided. Inclusion of a formula does not imply it is safe to handle.
Sources & local processing
- OpenStax Chemistry 2e, §7.6: VSEPR geometry; water 104.5° and ammonia 106.8° reference illustrations.
- CIAAW Abridged Standard Atomic Weights 2024: nominal element weights. Isotopic composition can vary; these are not exact constants.
- BIPM definition of the mole: Nₐ = 6.02214076 × 10²³ mol⁻¹ exactly.
- OpenStax §3.1, Formula Mass and the Mole Concept: formula-mass and amount relationships.
Geometry, amount calculation and JSON export run in your browser. No molecular data is uploaded. Inputs, assumptions, element values and sources travel with your downloaded report.
Sources checked 14 October 2026. Educational calculations are not a substitute for sample analysis or a measured molecular structure.