Lower-energy systems are more stable than higher-energy systems. Atoms achieve their ground state when their electrons occupy the lowest-energy orbitals. The aufbau principle states that electrons fill available orbitals with the least energy first. Notice that all orbitals in the same sublevel have the same energy. For example, the 2p_x, 2p_y, and 2p_z orbitals all have the same amount of energy just different orientations in space. Within each principle energy level, each orbital type has a different amount of energy. Note for example that the 2s orbital has less energy than the 2p orbitals. Watch how the aufbau principle is used when assembling a phosphorus atom. As orbitals and electrons are added to the structure the corresponding parts of the orbital diagram, electron configuration, and Lewis dot structure representing the arrangement will appear. Phosphorus has 15 electrons total. The lowest energy orbital, the 1s orbital will be filled first. The Pauli exclusion principle states that each orbital can hold a maximum of two electrons. Furthermore, when electrons are in the same orbital they must have opposite spin. Notice how the spin of electrons is represented in the orbital diagram. Arrows that point upward represent electrons with up spin, while arrows that point downward represent electrons with down spin. In the electron configuration, the superscript represents the number of electrons in the sublevel. Now that the 1s orbital is full, the next highest energy orbital, the 2s, will receive electrons. Just as before, two electrons of opposite spin will fill the 2s orbital. Now that the 2s orbital is full, the next highest energy orbitals, the 2p orbitals, will receive electrons. Hund’s rule states that when filling equal energy orbitals, electrons fill each orbital singly before filling orbitals with another electron already in them. There are three p orbitals of equal energy in the 2p sublevel. When electrons fill the 2p sublevel, they will fill each p orbital singly before filling an orbital with another electron in it. After each orbital of equal energy has been filled singly, electrons will begin to occupy orbitals that already have one electron in them. Notice that the electron configuration does not illustrate the individual orbitals of the arrangement. It is a more general representation of the arrangement, which only symbolizes the number of electrons in each sublevel of orbitals. For example, there are six electrons total that occupy the 2p orbitals, which make up the 2p sublevel, of this electron arrangement. So far, 10 electrons have been accounted for. Phosphorus has a total of 15 electrons. The remaining five electrons occupy the valence (or outermost and highest energy level). These are the electrons that will be represented in the Lewis valence electron dot structure. Valence electrons are the most important electrons in the electron arrangement because they are the electrons that are used for bonding. Energy level three contains the valence electrons of phosphorus. The 3s orbital, will receive electrons next. Two electrons of opposite spin will fill the 3s orbital. Finally, the 3p orbitals will receive electrons. As before, the electrons will fill orbitals of equal energy singly, before occupying an orbital with another electron already in it. Therefore, each of the three 3p orbitals will be occupied by a single electron. Now, all 15 electrons in the phosphorus atom have been accounted for. The orbital diagram, electron configuration, and Lewis valence electron dot structure are complete.