2s 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 orbital is seen halved at middle to show the axes of symmetry and the 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 2s electron orbital, 2 indicates that it is the second energy level, and s indicates that it is a single, spherical orbital. The 2 shell contains this 2s orbital and three 2p orbitals (not seen) at a slightly higher energy level.