3px electron orbital, illustration. An electron orbital is a region surrounding an atomic nucleus (not visible) where one or a pair of electrons is most likely to exist. The three 3p orbitals are made up of four lobes, two large outer ones and two smaller inner ones, centred on the nucleus. The orbital is seen transparent at middle to show the axes of symmetry and the spherical and planar nodes can be seen at right. Nodes are the regions in an atom with zero electron density and where the electron is least likely to exist. For the 3px electron orbital, 3 indicates that it is the third energy level, p indicates that the orbital is shaped like a dumbbell shape with two lobes, and x indicates the orientation of the orbital along the x-axis. The 3px orbital can accommodate up to 2 electrons. The 3 shell also contains a spherical, lower energy 3s orbital and five 3d orbitals at higher energy (not seen). In atoms, electrons fill the lower energy orbitals first. The 3p orbitals are only filled when the lower energy 1s, 2s, 2p and 3s orbitals are full. Hence, the 3p orbitals are full in element 18, Argon (1s2 2s2 2p6 3s2 3p6).