1s 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 axes of symmetry and the lack of nodes. Nodes are the regions in an atom with zero electron density and where the electron is least likely to exist. The 1s orbital is named as such because the 1 indicates that it is the lowest energy orbital and the s indicates that it is a single, spherical orbital. This means that the probability of finding the electron at any point within the orbital is uniform in all directions from the nucleus. The 1s orbital is filled in every element except hydrogen, which contains only one electron. This configuration is written as 1s1. In the next element, helium, the 1s orbital is full, containing two electrons. This is written as 1s2.