Energy levels: where electrons live
Electrons do not move just anywhere. They move around the nucleus in energy levels named K, L, M and N (or n = 1, 2, 3, 4). Think of seats in a theatre: the first row is small and the rows farther back have more seats. Level K holds at most 2 electrons, L at most 8, M at most 18 and N at most 32. The rule is 2 × n × n, so for n = 3 we get 2 × 3 × 3 = 18. Electrons fill the lowest level first, and only then the next one. The farther a level is from the nucleus, the more energy its electrons have. The outermost level never holds more than 8 electrons. Sodium has 11 electrons: 2 in K, 8 in L and 1 in M, written 2, 8, 1. A warning: for the first twenty elements, level M takes only 8 electrons before N starts, so potassium (19 electrons) is 2, 8, 8, 1, not 2, 8, 9.
| Level | n | Most electrons |
|---|---|---|
| K | 1 | 2 |
| L | 2 | 8 |
| M | 3 | 18 |
| N | 4 | 32 |
Cups and beads. Set out three paper cups in a row for K, L and M, and write 2, 8 and 18 on them. Use beads as electrons. For an atom with 11 electrons, fill K first, then L, and put the rest in M. Do the same for 3, 9 and 17 electrons. For 19 electrons, stop the M cup at 8 and put the last bead in a fourth cup, N.