Isotopes

Some elements occur in nature as a single kind of nuclide — fluorine, for instance, occurs essentially only as $^{19}_{9}F$. Most elements, however, occur naturally as a mixture of two or more nuclides that all share the same atomic number Z (so they are the same element and have identical chemical behaviour) but differ in their neutron count N and hence in mass number A. These different nuclides of one element are called isotopes of that element. Because all the isotopes of an element share the same Z, they occupy exactly the same position in the periodic table (the word 'isotope' literally means 'same place') and show essentially the same chemical properties, since chemical behaviour is governed by the number and arrangement of electrons, which is fixed by Z. Each isotope of an element that occurs in nature does so with its own characteristic, roughly constant natural abundance — chlorine, for example, is a natural mixture of about three parts $^{35}_{17}Cl$ to one part $^{37}_{17}Cl$, and the 'atomic mass' listed for chlorine in the periodic table (35.5) is really the abundance-weighted average across its isotopes, not the mass of any single chlorine atom. Carbon is a classic three-isotope example: $^{12}C$ makes up …