3s electron orbital, computer model. An electron orbital is a region around an atomic nucleus (not seen) in which one or a pair of electrons is most likely to exist. The orbital is sectioned to show the concentric spherical nodes within it. The orange areas are where the electron has a positive wave function, and the blue area is where it has a negative wave function. This is the lowest energy part of the third electron shell, which also contains three 3p orbitals and five 3d orbitals at higher energy levels. The 3 shell is only filled with electrons after the lower energy 1 and 2 shells have been filled, so the first element with a full 3s shell is magnesium (1s2 2s2 2p6 3s2).