Why do lone pairs look like purple clouds instead of atoms?

They are a visualization of non-bonding valence electron density occupying direction around the central atom. The simulator does not draw orbitals; it marks directions where lone pairs are assumed to repel bonded pairs more strongly than bond–bond repulsion alone would suggest.

Are the bond angles exact numbers like 109.5°?

Only for perfect tetrahedral, octahedral, or trigonal bipyramidal reference geometry. The scene uses symmetric unit vectors; real molecules (H₂O, NH₃, etc.) deviate because lone pairs are not identical to bonds and electronegativity pulls electron density. Compare this page to quantitative experiments or quantum calculations when precision matters.

Why does XeF₂ use two bonds and three lone pairs on xenon?

It is a hypervalent example where the steric number is five (trigonal bipyramidal electron geometry) but three equatorial sites are lone pairs so the two fluorines sit on opposite axial positions—giving a **linear molecular shape** for the nuclei.

Does this replace a full molecular viewer?

No. The Molecule Viewer lab shows real coordinates for specific compounds. Here the goal is to connect **counts** of bonding and lone pairs to **named geometries** and to practice AXₙEₘ reasoning independent of a particular bond length.

VSEPR Molecular Shapes (3D)

Valence Shell Electron Pair Repulsion (VSEPR) predicts three-dimensional arrangements of electron domains (bonding pairs and lone pairs) around a central atom. The steric number is the count of those domains; it sets the electron geometry (linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral for two through six domains). Molecular shape describes where the bonded atoms actually sit once lone pairs—often modeled as slightly larger, more repulsive regions—occupy some of those directions. This lab shows an idealized ball-and-stick plus translucent lone-pair lobes sketch: domain vectors are fixed to symmetric polyhedra, and lone pairs follow simplified textbook placement rules (for example, lone pairs prefer equatorial sites on a trigonal bipyramid to reduce 90° crowding; two lone pairs on an octahedron are shown trans to each other so the four bonds lie in a plane). Sliders change the AXₙEₘ counts; presets recall common textbook examples such as CO₂, H₂O, NH₃, CH₄, PCl₅, SF₆, XeF₂, and XeF₄. The model is pedagogical, not a quantum geometry optimization: real bond angles differ when electronegativity and π bonding matter, and some hypervalent molecules need molecular-orbital ideas beyond first-year VSEPR cartoons.

Who it's for: High school and first-year college general chemistry students learning VSEPR tables, AXE notation, and the distinction between electron geometry and molecular shape before Walsh diagrams or computational structure.

Key terms

VSEPR theory
Steric number
Electron geometry
Molecular shape
AXE notation
Lone pair repulsion
Trigonal bipyramid
Octahedral geometry
Square planar

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How it works

Valence Shell Electron Pair Repulsion (VSEPR) predicts electron geometry from the steric number (bonding + lone pairs) and molecular shape from where atoms actually sit. Rotate the 3D model to compare textbook angles with this idealized ball-and-lone-pair sketch (not a quantum geometry optimization).

Key equations

Steric number = (# bonding domains) + (# lone pairs) on central atom

Lone pair – lone pair > lone pair – bond > bond – bond repulsion (qualitative)

Trigonal bipyramid: lone pairs prefer equatorial sites (less crowding).

Frequently asked questions

Why do lone pairs look like purple clouds instead of atoms?: They are a visualization of non-bonding valence electron density occupying direction around the central atom. The simulator does not draw orbitals; it marks directions where lone pairs are assumed to repel bonded pairs more strongly than bond–bond repulsion alone would suggest.
Are the bond angles exact numbers like 109.5°?: Only for perfect tetrahedral, octahedral, or trigonal bipyramidal reference geometry. The scene uses symmetric unit vectors; real molecules (H₂O, NH₃, etc.) deviate because lone pairs are not identical to bonds and electronegativity pulls electron density. Compare this page to quantitative experiments or quantum calculations when precision matters.
Why does XeF₂ use two bonds and three lone pairs on xenon?: It is a hypervalent example where the steric number is five (trigonal bipyramidal electron geometry) but three equatorial sites are lone pairs so the two fluorines sit on opposite axial positions—giving a linear molecular shape for the nuclei.
Does this replace a full molecular viewer?: No. The Molecule Viewer lab shows real coordinates for specific compounds. Here the goal is to connect counts of bonding and lone pairs to named geometries and to practice AXₙEₘ reasoning independent of a particular bond length.

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Frequently asked questions