MolDraw › Chemistry tools › Build a Molecule
Build a Molecule
A virtual molecular model kit. Tap to add atoms, join them with single, double or triple bonds, and watch the formula update as you build. Valence rules stop impossible structures. Finished molecules are named, given a SMILES string and can be shown in 3D. Try challenge mode to test yourself.
Challenge mode
How to build a molecule
- Pick an atom from the kit (C, H, O, N, halogens, S, P, B) and tap the board to place it.
- With an atom selected (dashed ring), tap the board or tap the same atom again to add a bonded atom. Choose single, double or triple first.
- Join two atoms by selecting one and tapping the other. Tap a bond to change single → double → triple.
- Orange badges show how many bonds an atom still needs. Use +H fill to complete them with hydrogens.
- When every atom is satisfied, you get the formula, name and SMILES, and can view the molecule in 3D.
Valence rules used (neutral atoms)
| Atom | H, F, Cl, Br, I | O | N | B | C | S | P |
|---|---|---|---|---|---|---|---|
| Bonds | 1 | 2 | 3 | 3 | 4 | 2, 4 or 6 | 3 or 5 |
A double bond counts as 2 and a triple bond as 3. Charged species such as NH₄⁺ are not part of the neutral kit.
Challenges: 20 molecules to build
Start with water, methane and ammonia, move on to double and triple bonds (CO₂, N₂, ethyne), then isomers. Ethanol and dimethyl ether are both C₂H₆O, and the checker tells you when you have built the right formula with the wrong connectivity. It finishes with rings (cyclopropane).
Once you have built a molecule, explore its shape with the VSEPR calculator or rotate it in the 3D molecular model viewer.
Drawing structures for your notes? MolDraw is a free online chemical structure editor for molecules, reactions and lab diagrams.
Open MolDraw editorFrequently asked questions
How do I build a molecule online?
Choose an atom from the kit, tap to place it, then add bonded atoms or join atoms together. The builder checks each atom's valence, shows the formula as you go, and looks up the name when the molecule is complete.
Is this like the PhET Build a Molecule simulation?
It has the same idea, putting atoms together like a molecular model kit. It runs in any browser on phones and tablets, and adds double and triple bonds, a SMILES output, a name lookup and a 3D view.
Why can't I add another bond to an atom?
Each neutral atom has a fixed number of bonds: H 1, O 2, N 3, C 4. Once an atom's valence is full, the builder blocks extra bonds so you can only build real molecules.
What do the orange numbers mean?
They show how many more bonds an atom needs to complete its valence. Fill them with other atoms, add double bonds, or press +H fill to add hydrogens.
How does challenge mode work?
Pick a target molecule. When every atom has a full set of bonds, the builder compares your structure with the target. It even spots isomers that have the right formula but different connectivity.
Can I see my molecule in 3D?
Yes. Once it is complete and known to PubChem, press Show in 3D for a ball-and-stick model, or open it in the full 3D molecular model viewer.