Algodoo

Tutorial
1. Red is proton, white is neutron and blue is electron.
2. There are 16 of each particle. The white rings are electron shells.
3. Don't worry. You can always reset your atom by reloading the scene.
4. 16 is Sulfur, meaning you can only make isotopes until Sulfur-32.
5. The nucleus is inside the smallest white ring. That's where you can add protons and neutrons.
6. You can add any remake as long as it follows the Algobox rules.