# Why can a total of 18 electrons can be placed in the 3rd energy level?

The third energy level, n=3, contains the 3s, 3p, and 3d sublevels. The maximum number of electrons in the 3s sublevel is two, the maximum number of electrons in the 3p subsevel is six, and the maximum number of electrons in the 3d sublevel is ten. So $2 + 6 + 10 = 8$.