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A sealed flask weighs the same before and after a change.

This is Chapter 9 — Atomic Foundations of Matter.

What you will learn

• Nothing is lost — in a sealed flask, mass matches before and after.
• Always the same proportion — a compound's mass ratio stays fixed.
• Why those two hold — in a chemical change, atoms only rearrange.
• Weighing something you cannot see — each atom's mass, in units (u).
• Molecular mass — add a formula's masses together.
• When it is not molecules — a ratio, not a particle.
• Valency — each element's usual combining number.
• Writing the formula — swap two valencies, then cut down.

Quick catch-up

These earlier lessons help. Open one first if a line here is unclear.

• Joined in a fixed ratio — elements join to make a compound.
• An agreed short name — every element carries one written symbol.
• Which atom is it — the proton count decides the element.
• The outer shell decides — it decides how an atom joins.
• When something new turns up — test before and after a change.

One rule before every bench

Every bench in this track is a simulation. No bench asks you to mix, heat or handle anything.