The Quantum Revolution
From Bohr's planetary model to the modern electron cloud, explore the strange world of the quantum atom.
This tutorial builds on the discoveries of the 19th century.
← Go back to see how Dalton, Faraday, and Thomson set the stage.
Niels Bohr (1913)
To solve the problem of Rutherford's atom (why don't electrons spiral into the nucleus?), Niels Bohr proposed a radical idea: electrons can only exist in specific, fixed energy levels, or "shells." They can "jump" between these levels, but cannot exist in between.
Interactive Bohr Model
The electron is in its ground state (n=1). It must absorb energy to jump to a higher level.
The Quantum Mechanical Model (1926)
Bohr's model was a huge step, but it wasn't perfect. Schrödinger and Heisenberg developed a new model where electrons exist not in neat orbits, but in fuzzy "clouds of probability" called orbitals.
Interactive Orbitals
The Grand Finale: Electron Configuration
This is it! The quantum rules dictate how electrons fill these orbitals, which in turn defines every element's identity and chemical behavior. Use the controls to add electrons and watch the Periodic Table come to life.
Want the answer for any element?
This simulator is great for learning the rules, but if you need the configuration for any element instantly, use our powerful generator tool.
Use the Generator →The Rules of the Game
As you add electrons, you'll notice they follow three fundamental rules. Understanding these is the key to mastering electron configurations.
1. Aufbau Principle
From the German for "building up," this rule is like filling seats in a theater from front to back. Electrons always fill the lowest energy orbitals first.
2. Pauli Exclusion Principle
Think of an orbital as a bunk bed. It can hold a maximum of two electrons, but only if they have opposite spins (one "up," one "down").
3. Hund's Rule
This is the "bus seat rule." When on a bus (a subshell with multiple seats/orbitals), people will take an empty seat for themselves before they sit next to someone else. Electrons do the same to minimize repulsion.
You've Mastered the Single Atom. What's Next?
You now understand the structure of an individual atom. But atoms rarely exist alone. The next chapter in our story is to see how they interact with each other to form the molecules and compounds that make up our world.