Atoms and their energy levels

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.