Theoretical and Experimental Principles

Abstract

Isotopes are atoms whose nuclei contain the same number of protons but a different number of neutrons. The term " isotopes " is derived from Greek (meaning equal places) and indicates that isotopes occupy the same position in the periodic table. It is convenient to denote isotopes in the form m n E, where the super-script " m " denotes the mass number (i.e., sum of the number of protons and neutrons in the nucleus) and the subscript " n " denotes the atomic number of an element E. For example, 12 6 C is the isotope of carbon which has six protons and six neutrons in its nucleus. The atomic weight of each naturally occurring element is the average of the weights contributed by its various isotopes. Isotopes can be divided into two fundamental kinds, stable and unstable (radioactive) species. The number of stable isotopes is about 300; whilst over 1200 unstable ones have been discovered so far. The term " stable " is relative, depend-ing on the detection limits of radioactive decay times. In the range of atomic numbers from 1 (H) to 83 (Bi), stable nuclides of all masses except 5 and 8 are known. Only 21 elements are pure elements, in the sense that they have only one stable isotope. All other elements are mixtures of at least two isotopes. The relative abundance of different isotopes of an element may vary substantially. In copper, for example, 63 Cu accounts for 69 % and 65 Cu for 31 % of all copper nuclei. For the light elements, however, one isotope is predominant, the others being present only in trace amounts. The stability of nuclides is characterized by several important rules, two of which are briefly discussed here. The first is the so-called symmetry rule, which states that in a stable nuclide with low atomic number, the number of protons is approximately equal to the number of neutrons, or the neutron-to-proton ratio, N/Z, is approximately equal to unity. In stable nuclei with more than 20 protons or neutrons, the N/Z ratio is always greater than unity, with a maximum value of about 1.5 for the heaviest stable nuclei. The electrostatic Coulomb repulsion of the positively charged protons grows rapidly with increasing Z. To maintain the