ABSTRACT
The basics of atomic structure (see 4) should be appreciated in order to understand what is meant by radioactivity. Briefly, the nucleus of an atom, which contains positively charged protons and electrically neutral neutrons, is surrounded by negatively charged electrons. The number of protons in the nucleus (the atomic number) is fixed for each element, but most elements have atomic nuclei with a variable number of neutrons. The atomic mass of the atoms thus varies. The term isotope is used to refer to these different forms of the element. In other words, the isotopes of any named element are distinguished by the number of neutrons in the atomic nucleus. Elements typically have between two and seven isotopes, but some have more. For the majority of elements, one isotope is overwhelmingly predominant. To refer to an individual isotope, the mass number (i.e. proton number plus neutron number) is shown either as a superscript preceding the element symbol, as in 4C, or after the element name, as in carbon-14. The reasons why each element has a fixed number of isotopes are to be found in the ‘rules’ governing the combinations of protons and neutrons that confer stability on a nucleus.
