A single square holds four pieces of information:
| Position | What it is | Example (sodium) |
|---|---|---|
| Top | Atomic number = protons | 11 |
| Middle | Symbol | Na |
| Below | Name | Sodium |
| Bottom | Atomic mass | 22.99 |
Reading it well
- Atomic number is the identity of the element. Change it and you have a different element.
- Atomic mass is a weighted average across isotopes, which is why it is rarely a whole number. Round it to find the most common mass number.
- Neutrons = mass number − atomic number.
- Group (column) gives valence electrons for the main groups; period (row) gives the number of shells.
Finding what you need quickly
- Metal or non-metal? Left of the staircase is metal.
- Which group? That gives the likely ion charge: group 1 → , group 2 → , group 16 → , group 17 → .
- Full outer shell? Group 18 — expect no reaction.
Reference: The Periodic Table