Filling the shells
Sodium has 11 electrons. Where does each one go?
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Step 1 of 3
- The K shell fills first: 2 electrons.
- The L shell fills next: 8 electrons.
- The last electron goes in the M shell.
The idea
They go 2 in K, 8 in L and 1 in M.
Build an atom by adding one electron at a time. Each electron goes in the innermost shell that still has room. Innermost means nearest the nucleus. K takes 2. L takes 8. M takes only 8 in the first 18 elements. The outermost shell holds at most 8.
How electrons are spread over the shells is the electronic configuration. We write it as numbers in the order K, L, M. Sodium is 2, 8, 1. Check: 2 + 8 + 1 = 11.
| Z | Element | K, L, M |
|---|---|---|
| 1 | Hydrogen | 1 |
| 2 | Helium | 2 |
| 3 | Lithium | 2, 1 |
| 4 | Beryllium | 2, 2 |
| 5 | Boron | 2, 3 |
| 6 | Carbon | 2, 4 |
| 7 | Nitrogen | 2, 5 |
| 8 | Oxygen | 2, 6 |
| 9 | Fluorine | 2, 7 |
| 10 | Neon | 2, 8 |
| 11 | Sodium | 2, 8, 1 |
| 12 | Magnesium | 2, 8, 2 |
| 13 | Aluminium | 2, 8, 3 |
| 14 | Silicon | 2, 8, 4 |
| 15 | Phosphorus | 2, 8, 5 |
| 16 | Sulfur | 2, 8, 6 |
| 17 | Chlorine | 2, 8, 7 |
| 18 | Argon | 2, 8, 8 |
The 19th electron cannot join M, which already holds 8. It starts the N shell. Potassium is 2, 8, 8, 1.