Build a Molecule
3D molecular construction with real bond angles
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular geometry by minimizing electron-pair repulsion around each central atom. Count all electron domains (bonding pairs + lone pairs). Two domains → linear (180°). Three domains → trigonal planar (120°). Four domains → tetrahedral (109.5°). Five → trigonal bipyramidal. Six → octahedral. Lone pairs compress bond angles slightly because they occupy more space than bonding pairs. For example, water has 4 electron domains (2 bonding + 2 lone pairs) giving a tetrahedral electron geometry but a bent molecular shape with a 104.5° angle instead of 109.5°.
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Sign in →What is Build a Molecule?
Molecular geometry is the 3D arrangement of atoms in a molecule, determined by how electron pairs — both bonding and lone — push each other away to maximize separation. Ammonia smells sharp and dissolves readily in water because its trigonal pyramidal shape creates a net dipole; a flat version of NH₃ would behave completely differently. VSEPR (Valence Shell Electron Pair Repulsion) theory predicts this shape from a single rule: electron domains around a central atom adopt the geometry that minimizes repulsion. This simulation lets students choose atom buttons for manual construction and use preset molecules for water, methane, ethanol, and ammonia. Those examples make it easy to compare tetrahedral carbon centers, bent oxygen-centered water, and trigonal pyramidal ammonia while rotating the model freely. AP Chem 2.B.1 and 2.C.4 are the governing standards.
Common misconceptions
MisconceptionWater is linear because oxygen forms exactly two bonds, like CO₂.
CorrectThe molecular geometry follows the electron domains, not just the bonds. Oxygen in water has four electron domains: two bonding pairs and two lone pairs. Use the Water (H₂O) preset to inspect the bent arrangement, then compare it with the Methane (CH₄) preset, where carbon has four bonding domains and no lone pairs. The difference shows why visible bond count alone is not enough to predict shape.
MisconceptionLone pairs do not count when predicting molecular shape because they are not attached to an atom you can see.
CorrectLone pairs count as full electron domains in VSEPR and they exert more repulsion than bonding pairs. The Ammonia (NH₃) preset is the key comparison: nitrogen has three N-H bonds plus one lone pair, so the molecule is trigonal pyramidal rather than trigonal planar. Compare it with Methane (CH₄), where four bonding domains create a symmetric tetrahedral shape.
MisconceptionAll molecules with four bonds around the central atom have 109.5° bond angles.
Correct109.5° is the ideal angle only when all four domains are bonding pairs with no lone pairs, as in methane. Water and ammonia show why the rule needs domain counting, not just bond counting: lone pairs occupy space and compress nearby bonds. Use the Water (H₂O), Ammonia (NH₃), and Methane (CH₄) presets as a three-case comparison of two lone pairs, one lone pair, and no lone pairs.
MisconceptionIf two molecules contain the same atom types, they must have the same shape.
CorrectAtom identity matters, but connectivity and electron domains matter just as much. Ethanol contains carbon, hydrogen, and oxygen, yet it has tetrahedral carbon centers plus an oxygen-containing alcohol group rather than the same simple geometry as water. Use the atom buttons to identify each element in the model, then compare the Ethanol (C₂H₅OH) preset with Water (H₂O) to separate element composition from molecular shape.
How teachers use this lab
- Preset comparison opener: load Water (H₂O), Methane (CH₄), and Ammonia (NH₃), then ask students to identify the central atom, count electron domains, and explain why the three shapes differ.
- Manual construction vocabulary check: use the H, C, N, O, Cl, and S atom buttons as a quick element-recognition warmup before students describe how selected atoms become bonding partners in a 3D model.
- Lone-pair compression discussion: compare the Methane (CH₄) and Ammonia (NH₃) presets first, then add Water (H₂O) to show how increasing lone-pair count changes the observed geometry.
- Organic structure bridge: load Ethanol (C₂H₅OH) and have students locate the carbon backbone, the oxygen atom, and the terminal hydrogens, then connect local geometry to the alcohol functional group.
- AP Chem 2.B.1 retrieval practice: show one preset at a time with labels hidden by classroom discussion, asking students to name the molecular geometry before explaining it through VSEPR domain counting.
Frequently asked questions
Why is water's bond angle 104.5° instead of the tetrahedral 109.5°?
Water has four electron domains around oxygen: two bonding pairs and two lone pairs. Lone pairs are held by only one nucleus and spread out more than bonding pairs, exerting greater repulsion on the H-O bonds and pushing them together. Use the Water (H₂O) preset as the bent reference case, then compare it with Methane (CH₄), where four bonding domains around carbon produce the ideal tetrahedral arrangement.
What can I do with the atom buttons?
The H, C, N, O, Cl, and S buttons select the atom type for manual molecule construction. They are useful for connecting chemical symbols to model parts before students discuss bonding. Hydrogen often appears as a terminal atom, carbon commonly forms tetrahedral centers, nitrogen and oxygen introduce lone-pair examples, chlorine works as a terminal high-electronegativity atom, and sulfur gives a period-3 comparison point.
Which preset molecules are available in this HTML simulation?
The HTML preset buttons load Water (H₂O), Methane (CH₄), Ethanol (C₂H₅OH), and Ammonia (NH₃). These four examples cover a useful classroom spread: bent water, tetrahedral methane, an organic alcohol with carbon and oxygen centers, and trigonal pyramidal ammonia. Use the presets before manual construction so students have reliable reference structures for VSEPR comparisons.
Does this simulation cover AP Chem 2.B.1 and 2.C.4?
AP Chem 2.B.1 requires students to use VSEPR theory to predict molecular geometry, which is directly supported by comparing the Water (H₂O), Methane (CH₄), Ethanol (C₂H₅OH), and Ammonia (NH₃) presets. AP Chem 2.C.4 connects molecular structure to polarity, so water and ammonia provide strong polar-shape examples while methane offers a symmetric contrast. NGSS HS-PS1-1 and HS-PS1-3 are also supported through structure-property reasoning.
Why compare ethanol with water and methane?
Ethanol combines ideas students often learn separately. Its carbon atoms provide tetrahedral bonding examples like methane, while its oxygen-containing alcohol group connects back to water's polar O-H bonding and lone-pair behavior. Loading Ethanol (C₂H₅OH) after Water (H₂O) and Methane (CH₄) helps students see that a larger molecule can contain multiple local geometries rather than one single shape label for the whole structure.