Finding the period
Which period an element belongs to is decided by the number of energy levels its electrons are spread over.
Count the levels that actually hold electrons. That count is the period.
| Electrons are present in | The element is in |
|---|---|
| The first level only | Period 1 |
| The first and second levels only | Period 2 |
| The first, second and third levels only | Period 3 |
| The first, second, third and fourth levels only | Period 4 |
Reading it off a configuration
This makes the period the easiest thing in the chapter to find. You do not need the table in front of you — just count the numbers in the configuration.
Three examples
Ne — 2, 8 → two numbers → period 2
S — 2, 8, 6 → three numbers → period 3
Ca — 2, 8, 8, 2 → four numbers → period 4
Check those against the grid on the previous page. Neon does sit in row 2, sulphur in row 3 and calcium in row 4.