Lesson 9 of 11
The big picture
Show on a big screenEight ideas, one main idea. In a chemical change, atoms only rearrange. Most numbers here are a comparison or a sum.
| Idea | Remember | Example |
|---|---|---|
| Nothing is lost | Mass before a change equals mass after, in a closed system. | sealed flask: 250 g → 250 g |
| Always the same proportion | A compound's elements stay in one mass ratio. | water: H to O is 1 : 8 |
| Why those two hold | Atoms only rearrange. None are made or destroyed. | 8 H + 4 O → 4 H₂O |
| Weighing something you cannot see | Atomic mass compares an atom to carbon-12, fixed at 12 u. | oxygen = 16 u |
| Molecular mass | Add the formula's atomic masses, in u. | H₂O = 18 u |
| When it is not molecules | Ions in a ratio give a formula unit mass. | NaCl = 58.5 u |
| Valency | Each element in the table has one usual valency. | oxygen's valency is 2 |
| Writing the formula | Swap the valencies, then cut them down. | AlCl₃ |
Now open Chapter 9 — Atomic Foundations of Matter. Every idea in it is one you have already worked.