Build a Nucleus: Protons, Neutrons & Stability
Drag protons and neutrons to build atoms from hydrogen to calcium and check isotopes and stability.
Drag protons and neutrons from the buckets into the nucleus. See which element you built, its mass number and whether it is stable.
About the Build a Nucleus: Protons, Neutrons & Stability
Free build a nucleus: protons, neutrons & stability. Drag protons and neutrons to build atoms from hydrogen to calcium and check isotopes and stability. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for atomic & quantum, the build a nucleus: protons, neutrons & stability runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Drag protons and neutrons to build atoms from hydrogen to calcium and check isotopes and stability. Use it to explore atomic & quantum ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Build a Nucleus: Protons, Neutrons & Stability
- Press "+ Proton", "+ Neutron", "− Proton", "− Neutron" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Build carbon-12 and carbon-14.
- Build helium-3 and helium-4.
- Make an unstable nucleus and press Let it decay.
Key ideas you can learn
- The number of protons defines the element.
- Mass number = protons + neutrons.
- Isotopes have the same protons but different neutrons.
- Unstable nuclei decay.
Where this is used in the real world
Isotopes are used in carbon dating, medical imaging, cancer treatment and nuclear power.
Who is this simulation for?
Advanced high-school and university students in chemistry and physics, and curious learners who want to see how the atomic world behaves.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
What is an isotope?
An atom of the same element with a different number of neutrons.
What is beta decay?
A neutron changes into a proton (or the reverse), emitting an electron or positron.
Is the Build a Nucleus: Protons, Neutrons & Stability free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Build a Nucleus: Protons, Neutrons & Stability work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.