The maximum number of electrons that an energy level can hold is determined from the formula 2n^2 equals the total number, where n is the energy level. Thus, the first energy level holds 2 * 1^2 = 2 electrons, while the second holds 2 * 2^2 = 8 electrons.
Following the formula, the third energy level can contain 18 electrons, the fourth energy level can hold 32 electrons, the fifth energy level can hold 50 electrons, the sixth energy level can carry up to 72 electrons and the seventh energy level can contain 98 electrons. The energy levels, or shells, correspond with the letters K, L, M, N, O, P and Q respectively. Lower numbers correspond with the innermost energy level, while the seventh energy level is the outermost shell.
Each shell contains one or more subshells, each of which also can contain only a maximum number of electrons, determined by the formula 4l + 2, where l is the quantum number of the subshell. The s subshell can contain a maximum of two electrons, the p subshell can hold six electrons, the d subshell can carry 10 electrons, the f subsell can contain up to 14 electrons and the g subshell can contain a total of 18 electrons.