4pz 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 4p orbitals are made up of six lobes, two larger outer ones and four smaller inner ones, centred on the nucleus. The orbital is seen transparent at middle to show the axes of symmetry and the spherical nodes and planar node 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 4pz electron orbital, 4 indicates that it is the fourth energy level, p indicates that the orbital is shaped like a dumbbell with two lobes, and z indicates the orientation of the orbital along the z-axis. The 4px orbital can accommodate up to 2 electrons. The 4 shell also contains a spherical, lower energy 4s orbital and five 4d orbitals and seven 4f orbitals at a higher energy (not seen).