Isomeric shift levels E, followed by emission of photons. Atomic levels are a manifestation of the electromagnetic interaction between electrons and nuclei. The energy levels of two atoms the nuclei of which are different isotopes of the same element are shifted one with respect to the other, despite the fact that the electric charges Z of the two isotopes are identical. This is so because isotopes differ by the number of neutrons and therefore the masses and volumes of two isotopes are different; these differences give rise to the isotopic shift on atomic spectral lines. In the case of two nuclear isomers
Atomic number was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive charge now was realized to come entirely from a content of 79 protons. After 1932, therefore, an element's atomic number "Z" was also realized to be identical to the proton number of its nuclei. The conventional symbol "Z" possibly comes from the German word (atomic number). However, prior to 1915, the word "Zahl" (simply "number") was used for an element's assigned number in the periodic table. Each element has a specific set of chemical properties as a consequence of the number of electrons present in the