Electrons in different shells have different amounts of energy (shells).Each shell or energy level is given a principal quantum number from n = 1 to n = 7.The number of electrons that can exist in each shell or energy level is 2n2The first energy level can have up to 2 electrons, the second energy level can have up to 8 electrons and the third energy level can have up to 18 electrons.Each energy level is divided into sub-shells or sub-levels.The first energy level has only an s sub-level, the second has both an s and a p sub-level, the third has an s, p and d sub-level and the fourth has a s, p, d and f sub-levels.