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How are electrons placed in shells around the nucleus?
Electrons tend to arrange themselves around nuclei so that they have the lowest possible energy. They would all like to get into the lowest energy level, sometimes called the K-shell, but are prevented from doing so by some rules that pop up in quantum mechanics. You can see how electrons are arranged in a particular atom by taking a look at our Periodic Table of Elements. Let's use oxygen as an example. Oxygen's electron configuration is:
| 1s2 |
| 2s2 2p4 |
This means that the first energy level (the K-shell) contains 2 electrons, both in sub-shell s, and that the second energy level (the L-shell) contains 6 electrons, 2 in sub-shell s and 4 in sub-shell p. As more electrons are added, higher energy levels with more sub-shells become filled. Here is a list of the first few energy levels, their sub-shells, and the maximum number of electrons that they can contain:
| Energy Level | Sub-shells | Number of Electrons |
| 1 (K-shell) | s | 2 |
| 2 (L-shell) | s, p | 8 (2 in s, 6 in p) |
| 3 (M-shell) | s, p, d | 18 (2 in s, 6 in p, 10 in d) |
| 4 (N-shell) | s, p, d, f | 32 (2 in s, 6 in p, 10 in d, 14 in f) |
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