ABSTRACT
The nucleus of any atom consists of a mixture of protons and neutrons (see Chapter 2); a simple nuclear model therefore consists of Z protons and N neutrons, making a total of A nucleons. Each element ‘X’ can therefore be defined uniquely in terms of these three components:
where Z is the atomic number (proton number), N the neutron number and A the mass number (Z N). Some common elements – iodine, potassium, and carbon – are described below using this coding:
In practice a nucleus can be identified by reducing these parameters to only Z and A. The atomic number Z is redundant, since the element name defines the value of Z; change the number of protons and the element changes. Therefore the mass number A is commonly used as a single identifier: 131I, 40K, 14C. This is sufficient to identify a nuclear species or nuclide exactly. This model of the nucleus, using only neutrons and protons, fits all practical requirements in nuclear medicine. In the early days of chemistry most naturally occurring elements, e.g. iron, were found to have atomic mass values that were not whole numbers. This
problem was solved when it was found that most chemical elements consist of a mixture of two or more species having different neutron numbers; iron has four stable species making up the common metal. These are shown in Fig. 15.1.
