Periods and groups
The periodic table is a grid, and its two directions have names.
- The horizontal rows are called periods.
- The vertical columns are called groups.
At this level you study only the part of the table holding the elements with atomic numbers 1 to 20. Here it is: periods run across, groups run down.
| Period | I | II | III | IV | V | VI | VII | VIII / 0 |
|---|---|---|---|---|---|---|---|---|
| 1 | H | He | ||||||
| 2 | Li | Be | B | C | N | O | F | Ne |
| 3 | Na | Mg | Al | Si | P | S | Cl | Ar |
| 4 | K | Ca |
Figure 3.4 — the part of the modern periodic table holding elements 1 to 20.
Two questions follow from this grid, and the next two pages answer them in turn: what decides which row an element goes in, and what decides which column?