High Quality Content by WIKIPEDIA articles! X-ray notation is a method of labeling atomic orbitals that grew out of X-ray science. IT is still traditionally used with most x-ray spectroscopy techniques including AES and XPS. IN X-ray notation, every principal quantum number is given a letter associated with it. AN atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. THis function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus. THese functions may serve as three-dimensional graph of an electron s likely location. THe term may thus refer directly to the physical region defined by the function where the electron is likely to be.