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A steel tumbler. A pinch of salt. The air in this room.

All three are built of atoms. Is an atom the smallest piece of matter? Or does it have parts inside?

Look at this atom. Its centre is the nucleus. Small particles called electrons move around it in layers called shells.

This is Chapter 8 of your Class 9 book, Exploration. It is called Journey Inside the Atom.

What you will learn

• The piece has parts — electrons show that atoms can be divided.
• Something with a charge — an atom is neutral overall.
• Where the mass sits — a tiny, dense nucleus in empty space.
• A picture that had to change — four models, one after another.
• Shells — electrons sit in shells with limited room.
• Which atom is it — atomic number and mass number.
• Filling the shells — electrons fill in order, K then L then M.
• The outer shell decides — it decides what an atom does.

Quick catch-up

These help, though none is required.

• Pieces too small to see — matter is made of particles.
• What this picture explains — a model must explain results.
• Substances that will not come apart — chemistry cannot split elements.
• An agreed short name — every element has a written symbol.

One rule before every lab

Every lab in this track is a simulation. Nothing here asks you to rub, heat or mix anything. Nothing is fired at anything real.