The first twenty elements
Here are the electron configurations of the first twenty elements — the ones you study at this level.
| Element | Symbol | Atomic number | K | L | M | N |
|---|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | |||
| Helium | He | 2 | 2 | |||
| Lithium | Li | 3 | 2 | 1 | ||
| Beryllium | Be | 4 | 2 | 2 | ||
| Boron | B | 5 | 2 | 3 | ||
| Carbon | C | 6 | 2 | 4 | ||
| Nitrogen | N | 7 | 2 | 5 | ||
| Oxygen | O | 8 | 2 | 6 | ||
| Fluorine | F | 9 | 2 | 7 | ||
| Neon | Ne | 10 | 2 | 8 | ||
| Sodium | Na | 11 | 2 | 8 | 1 | |
| Magnesium | Mg | 12 | 2 | 8 | 2 | |
| Aluminium | Al | 13 | 2 | 8 | 3 | |
| Silicon | Si | 14 | 2 | 8 | 4 | |
| Phosphorus | P | 15 | 2 | 8 | 5 | |
| Sulphur | S | 16 | 2 | 8 | 6 | |
| Chlorine | Cl | 17 | 2 | 8 | 7 | |
| Argon | Ar | 18 | 2 | 8 | 8 | |
| Potassium | K | 19 | 2 | 8 | 8 | 1 |
| Calcium | Ca | 20 | 2 | 8 | 8 | 2 |
Table 3.2 — electron configurations of the elements with atomic numbers 1 to 20.
What this shows
Read down the columns rather than across the rows and the pattern appears. The K column fills and stops. The L column then counts 1 to 8 and stops. The M column starts counting again — and this repeating is exactly what the periodic table in the next section is built on.