Isotope Isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in mass number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. The term isotope is formed from the Greek roots isos (ἴσος "equal") and topos (τόπος "place"), meaning "the same place"; thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table. The number of protons within the atom's nucleus is called atomic number and is equal to the number of
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