The principal energy levels contain sublevels designated by the letters s, p, d, f. Just as successive shells have higher energy, successive subshells also have higher energy. The region of space where there is a significant probability of finding a particular electron is known as an orbital. It is sometimes referred to as an electron cloud. The shape of this orbital is illustrated as a solid sphere, All of the orbitals in a specific subshell have the same energy. For atoms with more than one electron, the enery of the subshells within a shells is not the same and increases in order: s < p < d < f. Notice that the 4s energy sublevel is lower in energy than the 3d. An electron spins on tis own axis in one of only two direction, clockwise or counterclockwise. As a result, only two electrons can occupy the same orbital contains two electrons. The maximum number of electrons that can exist in the sublevels are tow in the s orbital, six in the three p orbital, ten in the five d orbital, and fourteen in the seven f orbitals. This principle is identified as Paul's exclusion princple. Not all principal energy levels contain each type of sublevel. The following are rules to determine what types of sublevels occu in any given enegy level and the maximum number of electrons possible in that energy level. (1) No more than two electrons can occupy one orbital. (2) Electrons occupy the lowest possible energy sublevels; they enter a higher sublevel only when the lower sublevels are filled. (3) Orbitals in a given sublevel of equal energy are each occupied by a single electron before a second electron enters them.

Hund's rule tells us that when electrons have more than one equivalent orbital available, they will half-fill each of the equivalent orbitals before filling the second half of each. Oxygen has eight electrons, and tis electronic configuration is 1s2 2s2 2p4, In accordance with Hund's rule, the four 2p electrons half fill each orbital before pairing up. The spdf atomic orbitals have definite orientations in space. They are represented by particular spatial shaped. The s orbitals are spherical and the p orbitals are dumbbell-shaped.