Chemistry · Chapter 02
⚛️ Atomic Structure
Protons, neutrons, electrons.
💡 Inside Every Atom
An atom has a tiny dense nucleus at its centre, made of protons (+) and neutrons (no charge). Around the nucleus, electrons (−) move in shells.
Shells are filled in a specific order:
• 1st shell (K) → up to 2 electrons
• 2nd shell (L) → up to 8 electrons
• 3rd shell (M) → up to 18 electrons
The atomic number = number of protons. The mass number = protons + neutrons. Hydrogen has 1 proton. Carbon has 6. The outer shell decides how the atom reacts.
Atomic structure — protons, neutrons, electrons in shellsWikimedia Commons / CC BY-SA 3.0
Bohr model of atom — electrons in fixed circular orbitsWikimedia Commons / CC BY-SA 3.0
🎬
Electron Shells in Action
AnimationElectrons orbit in shells. Try different elements below — see how shells fill up.
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Pick an Element
ExperimentChoose any element — see how its electrons fill into shells (2, 8, 18 rule).
ElementSodium (Na)
Configuration2, 8, 1
The number in the outermost shell decides how the atom bonds with others.