Shells
An electron cannot be at any distance from the nucleus. It stays in an allowed shell.
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The idea
Bohr's shells have letters and numbers. K is n = 1. L is n = 2. M is n = 3. N is n = 4. The letter n is the shell's number, counted from the nucleus.
- An electron stays in a shell. It is not found between two shells.
- Each shell has a fixed energy. The electron does not lose it while it stays.
- K is nearest the nucleus and has the least energy. Energy rises with distance.
- To change shell, an electron must take in or give out energy. The amount is fixed.
A shell can hold at most 2n² electrons. For K, that is 2 × 1² = 2. For L it is 2 × 2² = 8. For M it is 2 × 3² = 18.
The outermost shell of any atom holds at most 8 electrons. When K is the outermost shell, it holds at most 2.
In the first 18 elements, electrons fill from the inside out. K comes first, then L, then M. L starts to fill only when K is full.