Build an Atom: free chemistry simulation
Add protons, neutrons and electrons. Find the element, its charge and mass number, and whether it is stable.
▶ Open the simulationFree, no sign-up. Works on phones, tablets and laptops.
The science
The number of protons decides which element an atom is. Neutrons add mass and change the isotope, and electrons decide the charge: more electrons than protons makes a negative ion. Only some combinations of protons and neutrons make a stable nucleus.
Mass number A = protons + neutrons
Things to try
- Build carbon-12 and carbon-14 and compare their stability.
- Add an extra electron to make a negative ion.
- Find the element with 8 protons.
Topics covered
protons neutrons electrons isotope ion element mass number
How to use it
Open the simulation, then change the experiment live with the sliders, checkboxes and buttons in the side panel. Drag objects on screen where it makes sense, and use Pause, Step, Speed and Reset in the bottom bar to study every moment.
Frequently asked questions
What does the Build an Atom simulation show?
The number of protons decides which element an atom is. Neutrons add mass and change the isotope, and electrons decide the charge: more electrons than protons makes a negative ion. Only some combinations of protons and neutrons make a stable nucleus.
What is the key equation?
Mass number A = protons + neutrons
Is the Build an Atom simulation free?
Yes. It runs free in any web browser on phones, tablets and laptops, with no sign-up or download.
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