No, it is not possible for the M shell (the third energy level) to accommodate 18 electrons. The maximum number of electrons that can occupy a given shell can be determined using the formula 2n^2, where n represents the principal quantum number of the shell.
For the M shell (n = 3), the maximum number of electrons it can hold is 2(3)^2 = 2(9) = 18. However, it's important to note that the M shell is actually made up of three subshells: the 3s, 3p, and 3d subshells. The 3s subshell can hold a maximum of 2 electrons, the 3p subshell can hold a maximum of 6 electrons, and the 3d subshell can hold a maximum of 10 electrons.
Therefore, the total number of electrons that can occupy the M shell is 2 + 6 + 10 = 18. So, it is possible to have a maximum of 18 electrons distributed among the subshells of the M shell, but not all in a single subshell.