The Molecule Shapes Basics simulation from PhET lets you explore molecule shapes by building molecules in 3D. You can add atoms and watch the bonds move apart, which shows that molecular shape comes from repulsion between electron groups. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for the chemical bonding and molecular shape lessons in Tamil Nadu State Board (Samacheer Kalvi) Chemistry.
What is the Molecule Shapes Basics simulation?
This interactive simulation lets you build molecules in 3D and see how the shape changes as you add atoms. You can turn the molecule around, watch the bonds spread out and notice that bonds are not fixed in place. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to high school
Molecular shape, VSEPR, bonds and bond angles
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to recognize that molecular shape results from repulsion between atoms, and to recognize that bonds are not fixed in place and can rotate in response to those repulsions.
Launch the Molecule Shapes Basics simulation online
Tap or click inside the player and start exploring. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Molecule Shapes Basics simulation, step by step
- Predict first. Before you add an atom, say what shape you expect the molecule to take.
- Build a molecule. Add atoms to a central atom, one at a time, and watch the shape change.
- Turn it around. Rotate the model in 3D and look at it from different sides.
- Look at the bond angles. Notice how the bonds move to be as far apart as possible.
- Compare shapes. Change the number of atoms around the center and write down each shape you see.
Tip for students: keep a table of the number of bonded atoms and the shape you see. The pattern is easy to spot.
The idea behind the simulation: VSEPR
Electron groups repel, so they spread out
VSEPR stands for valence shell electron pair repulsion. Bonds and lone pairs around a central atom are all regions of negative charge, so they repel each other and move as far apart as they can. This gives each molecule its shape. Lone pairs take up more room than bonds, so they squeeze the bond angles a little smaller.
Worked example: Methane (CH₄) has 4 bonds and no lone pairs, so its shape is tetrahedral with bond angles of 109.5°. Ammonia (NH₃) has 3 bonds and 1 lone pair, so it is trigonal pyramidal with angles of about 107°. Water (H₂O) has 2 bonds and 2 lone pairs, so it is bent with an angle of about 104.5°.
Molecular shape key terms explained
These are the words you will meet while using the Molecule Shapes Basics simulation, with simple meanings.
VSEPR
A model that predicts molecular shape from the repulsion between electron groups.
Molecular geometry
The 3D arrangement of the atoms in a molecule.
Bond angle
The angle between two bonds that meet at the central atom.
Bond
A link between two atoms that shares electrons.
Lone pair
A pair of electrons on the central atom that is not in a bond.
Bond order
The number of bonds between two atoms: single, double or triple.
Central atom
The atom in the middle that the other atoms are bonded to.
Molecule
Two or more atoms joined by bonds.
Common mistakes: Forgetting to count lone pairs when choosing the shape, drawing molecules flat when they are 3D, and thinking that bonds are fixed in position.
Try the molecule shape finder (practice what you explore)
Choose the number of bonded atoms and lone pairs on the central atom. The finder shows the shape, the bond angle and an example. Then tap a “what if” button.
What happens when you change a variable
| Change | Effect |
|---|---|
| 2 bonds, no lone pairs | Linear, 180° |
| 3 bonds, no lone pairs | Trigonal planar, 120° |
| 4 bonds, no lone pairs | Tetrahedral, 109.5° |
| Swap a bond for a lone pair | The shape changes and the bond angles shrink a little |
| Add more electron groups | The groups spread out in 3D to stay far apart |
Classroom activities for teachers
The Molecule Shapes Basics simulation fits a single 40-minute period.
- Predict and test (10 min): students predict the shape for a given number of atoms, then build it.
- Shape table (10 min): students build molecules with 2, 3 and 4 atoms and record the shape and the angle.
- Turn and look (10 min): students rotate the model and sketch the 3D shape.
- Exit ticket (10 min): students explain in two sentences why the bonds spread out.
Looking for the matching chapters? Browse the Chemistry books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the chemical bonding lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of molecular shape
Score: 0 / 5
Molecule Shapes Basics simulation: frequently asked questions
Is the Molecule Shapes Basics simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does it work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
What is VSEPR theory?
VSEPR theory says that the electron groups around a central atom repel each other and move as far apart as possible. This decides the shape of the molecule.
Why is water bent and not linear?
The oxygen atom in water has 2 bonds and 2 lone pairs. All four groups spread out in 3D, so the two bonds are pushed into a bent shape with an angle of about 104.5°.
The Molecule Shapes Basics simulation turns invisible 3D shapes into something you can build, turn and see. Explore it, take the quiz above, and keep building until predicting a molecule’s shape feels easy.