Question: how many electrons in each principal energy level?
Answer: Each principal energy level can contain up to 2n2 electrons, where n is the number of the level. Thus, the first level can contain up to 2 electrons, 2(12) = 2; the second up to 8 electrons, 2(22) = 8; the third up to 18, 2(32) = 18; and so on.