2. The structure of atom and nucleus - nuclides, isotopes, isobars – nuclide charts and tables

Chapter 2 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

The atom consists of a small nucleus and of electrons surrounding it. The diameter of the nucleus ranges between 1.5·10^-15 m and 10·10^-15 m or 1.5-10 fm (femtometers) whereas the diameter of whole atom is 1-5·10^-10 m or 1-5 Å (angstrom) or in SI-units 0.1-0.5 nm (nanometers). Great majority of the mass, however, is in the nucleus. Compared to density of an atom the density of nucleus is huge at 1017 kg/m³.

Atomic nucleus consists of protons (p) and neutrons (p), together these nuclear particles are called nucleons. Protons are positively charged, having a charge of one unit (+1) while neutrons are neutral having no charge. The number of protons determines the chemical nature of atoms, i.e. of what elements they are. The number of protons (Z) is called the atomic number and it is characteristic for each element. The number of neutrons is designated by letter N and the sum of protons and neutrons is called the mass number (A). Thus A (mass number) = Z (proton number) + N (neutron number).

In the nucleus the force that binds the protons and neutrons is the nuclear force that is far stronger force than any other known force (gravitation, electric, electromagnetic and weak interaction forces). The range of the nuclear force is very short; the space where it acts is approximately same as the volume of the nucleus. The nuclear force is charge-independent, so the n-n, p-p and n-p attraction forces are of same strengths, and short range means that nucleons sense only their nearest neighbors. Figure II.1. shows the potential diagram of a nucleus, i.e. the potential energy as a function of the radius of the nucleus. In the figure, the range of nuclear force can be seen as potential well outside of which there is a positive electric layer, potential wall, due to positive charges of protons in the nucleus. Any positively charged particles entering the nucleus have to surpass or pass this potential wall. For a neutron, with no charge, it is easier to enter the nucleus since it does sense the potential wall.

Figure II.1. Potential diagram of an atomic nucleus.

Electrons (symbol e or e^-) surrounding the nucleus are located in shells (Figure II.2.). Electrons closest to the nucleus are located on the K shell and they have the highest binding energy, which decreases gradually on outer shell L, M, N and O. The charge of the electron is equal but opposite to that of proton, one negative unit (-1). To preserve its electrical neutrality the atom has as many electrons as there are protons.

Figure II.2. Atomic nucleus and the electron shells.

Nuclide is defined as an atomic nucleus with a fixed number of protons (Z) and a fixed number of neutrons (N). Thus, also the mass number (A) is fixed for a certain nuclide. Nuclides are presented as elemental symbols having the atomic number (Z) on the lower left corner and the mass number (A) on the upper left corner.

¹²₆C (carbon-12) ¹⁸₈O (oxygen-18) ³⁵₁₆ S (sulphur-35)

Since the atomic number is already known from the elemental symbol, it is usually left away and the nuclides are presented as follows ¹²C, ¹⁸O and ³⁵S. Sometimes, especially in the older literature, the nuclides are marked in the following way C-12, O-18 and S-35.

With respect to stability, the nuclides can be divided into two categories:

• stable nuclides
• unstable, radioactive nuclides, shortly radionuclides

Isotopes are defined as nuclides of the same element having different number of neutrons. Thus the mass number of isotopes varies according to the number neutrons present. For example, ¹²C and ¹³C are isotopes of carbon, the former having six neutrons and the latter seven. These two are the stable isotopes of carbon with the natural abundances of 98.9% and 1.10%, respectively. In addition to these carbon has several radioactive isotopes, radioisotopes, with mass number of ⁹C – ¹¹C and ¹⁴C – ²⁰C, of which the best known and most important is ¹⁴C.

Radioisotope and radionuclide terms are often incorrectly used as their synonyms. Radionuclide, however, is a general term for all radioactive nuclides. We may, for example, say that ¹⁴C, ¹⁸O and ³⁵S are radionuclides, but we not should say ¹⁴C, ¹⁸O and ³⁵S are radioisotopes since radioisotope always refers to radioactive nuclides of a certain element. So, we may say, for example, that ¹⁴C, ¹⁵C and ¹⁶C are radioisotopes of carbon. The two heavier isotopes of hydrogen ²H and ³H are most often called by their trivial names deuterium and tritium, designated as D and T.

Isobar, as will be seen later in context of beta decay, is an important term also. Isobars are defined as a nuclide having a specific mass number, such as ³⁵Ar, ³⁵Cl, ³⁵S and ³⁵P are isobars.

A graphical presentation, where all nuclides are presented with neutron number as x-axis and proton number as y-axis (or the other way round), is called a nuclide chart (Figure I.3.). Stable nuclides in the middle part are often marked with black color. The radioactive nuclides are located on both sides of the stable nuclides, neutron rich on right side and proton-rich on the left. In this kind of presentation, the elements are listed on vertical direction while the isotopes for each element are on horizontal lines. The isobars, in turn, can be seen as diagonals of the chart. For each nuclide, some important nuclear information, such as half-life, is given in the boxes. More detailed nuclear information can be found in nuclide databases, some of which are freely available in the internet, such as http://ie.lbl.gov/toi/.

Figure II.3. Part of a nuclide chart.