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NUCLEAR CHEMISTRY by E

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NUCLEAR CHEMISTRY by E Chapter 18 NUCLEAR CHEMISTRY by E. BRoDA Radiochemical Department, Institute 0/ Physical Chemistry, University 0/ Vienna, Allstria For the purpose of this Guide, "Nuclear Chemistry" will be taken as being concerned with the various ways in which the chemical properties of matter are affected by nuclear reactions and by events resulting from these reactions. NUCLEAR CONSTITUTION The now accepted ideas ab out the internal structure of the atom were developed just before the first world war by two physicists, then collaborating in Manchester, the New Zealander Ernest Rutherford and the Dane Niels Bohr. According to their theory, every atom can be pictured as a kind of minute planetary system. In the centre of the atom, there is a positively charged nucleus. A number of negatively charged electrons circle around this nucleus. The force holding the electrons within the atom is electrical attraction (Coulomb force) while in the solar system the planets are, of course, held by gravity. The positive charge of the nucleus is entirely due to the protons contained in it. Each proton contributes I elementary unit of electric charge (1.6 . 10-19 coulomb); the number of unit charges (" charge number ") of a given nucleus indicates immediately the number of protons within the nucleus. The charge of each electron is identical in magnitude with that of a proton, but is negative. Therefore, an electrically neutral atom contains as many electrons as protons; the number of electrons in the neutral atom is determined by the charge number of the nucleus. It was later (1932) found by Chadwick in Cambridge that most nuclei contain, in addition to the protons, electrically neutral particles, so­ called neutrons, which have practically no interaction with electrons. A comprehensive term for the two kinds of particles occurring in nuclei, i.e. protons and neutrons, is "nucleons". The number of nucleons within an atom is called its " mass number ". The capacity of each neutral atom for chemical interaction with other atoms-i.e. its chemical properties-is determined to a large extent by the number of its electrons and therefore ultimately by the charge number of the nucleus. Because of its profound importance for the behaviour of the atom, the charge number is also known as " atomic number". Thus, there are as many different kinds of atoms behaving differently chemically, as there are different charge numbers of nuclei, or atomic numbers. (This statement will require qualification later.) For ex am pIe, all atoms with charge number I 335 (1 proton in the nucleus) behave identically, namely as atoms of the " element" hydrogen. Similarly, the element helium is formed by the atoms with charge number 2, iron has charge number 26, and uranium 92. Uranium is the natural element with the highest atomic number. Although it is not important to know such figures by heart, it is instructive to compare the masses of the ultimate constituents of the atoms. The masses of the two kinds of nucleons are nearly the same, the neutron being only 0.13 per cent heavier than the proton. However, the proton is 1836 times heavier than the electron. The mass of the atom, then, is concentrated to an overwhelming extent within the nucleus. Further, it is important to point out that the particles take up only a small fraction of the total volume of the atom. The radius of a nucleus lies (depending on the number of nucleons, i.e. its mass number) between about 1.4· 10-13 (proton) and 6.5 . 10-13 cm. This is to be compared with the radius of the atom as a whole which, e.g. in the ca se of hydrogen, amounts to 0.53 . 1O-8cm. Thus in hydrogen the radius of the atom is about 38,000 times larger than the radius of the nucleus, and the position is qualitatively similar in other elements. Consequently, all atoms may be considered as largely empty, though in the parlance of modern physical theory such a statement would not be regarded as quite exact. On the other hand, it follows that the density of nuclear matter is incredibly high. From the radius (1.4' 10-13 cm) and the mass (1.67 . 10-24 g) of the proton it is readily calculated that the density is about 1.4· 1014 times higher than that of the standard substance, water. ELEMENTARY PARTICLES In addition to those already mentioned (proton, neutron, electron), more elementary particles-among them various kinds of " mesons "-have been discovered by the physicists since the nineteen thirties in experiments with cosmic rays and with man-made e1ectrical machinery. Each particle either carries positive or negative unit charge or it is neutral. The particles are short­ lived and transform spontaneously within a small fraction of a second with the ultimate production of stahle (protons, electrons, neutrinos and photons) or nearly stahle (neutrons) particles. At present, it is not known why the masses of the various elementary particles have the particular values shown in Table 1. TABLE 1. SO ME PROPERTIES OF IMPORTANT ELEMENTARY PARTICLES Relative Approximate Charge Partic1e Relative Mass* (Charge Half-Life (Mass 0/ Proton = 1) 0/ Proton = 1) Proton ...................... 1 1 stable Neutron ..................... 1.001 3 0 17 min. Electron (negative) ............ 0.000544 7 -1 stable Electron (positive) . 0.0005447 1 stable n-Meson (neutral) ............ 0.378 2 0 < 10-15 sec. n-Meson (positive) ............ 0.3659 1 2.6 X 10-8 sec. Neutrino . probably 0 0 stable Photon ...................... ! 0 0 stable * The masses given refer to the ffee particles at rest. Einstein's theory of relativity shows that actually the mass depends on the velocity of the partiele. 336 The photons, i.e. the quanta of electromagnetic radiation (visible light, UV light, X-rays etc.) are also considered as elementary particles. They invariably move with one and the same velocity e (e 3 X 1010 cm/sec.). The charge of the photon is zero. The mass of the photon is correlated with its energy by the famous general Einstein relationship : E= me2 The energy in turn is detennined by the frequency ofthe radiation in accordance with the law discovered by Max Planck in Berlin in 1900: E hv Because frequency and wave length I, are universally connected by: e = VA the Planck law can also be written as: E = he/I, and the Einstein law for photons as: In = h/el' Thus the mass of the photon-moving with its characteristic velocity-has no unique value but is determined by its frequency, or, what comes to the same, by its wave length. The short-lived elementary particles cannot be normal constituents of atoms in the same sense as nucleons or electrons, and in general teachers of chemistry will not wish to discuss their nature or physical role. NUCLEAR FORCES How is it that the nuclei keep together at all? What acts as " nuclear glue"? While the electrons are kept within the atoms by electrical (Coulomb) forces, obviously such forces cannot be invoked to explain the mutual attraction of the nucleons. Being neutral, the neutrons do not exert any electrical forces at all. The protons, which all carry charges of identical sign (positive), can only repel one another. Nevertheless, the attractive (binding) fOl·ces within the nuclei are very strong. Numerical values will be given below. It will be necessary for the teacher to emphasize strongly the existence and the strength of these specific nuclear forces and to underline their non­ electric nature. Though it will hardly be possible to explain their origin, the teacher should be clear about the principle involved. It is the view of theore­ tical physics that nuclear attraction (nuclear cohesion) is mediated by TC-mesons. All nucleol1s have a tendency to emit TC-mesons. A very short time after emission, the (positively charged or neutral) meson is reabsorbed by the parent nucleon, or else it is absorbed by a neighbouring nucleon. The transfer of a neutral meson does not change the nature of the nucleons involved, but on the loss of a positive meson a proton is converted into a neutron, and on the gain of such a meson a neutron is converted into a proton. Through this exchange between the nucleons, the mesons act as a kind of bridge or glue, and the calculations of the theoretical physicists show that a strong attractive force results from this type of interaction. The attraction is independent of the charge of the nucleon. It will be seen later that the Coulomb repulsion between the protons, which is superimposed on the specific nuclear force, may strongly affect the properties of nuclei, particularly nuclei of high charge. 337 NUCLEAR ENERGY UNITS The energy needed for the separation of an electron from an atom, i.e. for the removal of this electron, is often modest. This energy is called the " ionization energy" because in the process the atom is transformed into a positive ion. For instance, in the case of some atoms (rubidium or cesium) the relatively small energy of a photon of visible light suffices for ionization, and the phenomenon in this ca se is known as the photo-electric effect, and is used in photo-electric cells. In other cases, much more energy may be required for the removal of an electron. This is true in particular for electrons which circle in lower shells, i.e. in orbits ne ar the nucleus. Nevertheless, the binding energy of the electrons in atoms is always much less than the binding energy of nucleons in nuclei. That means that the attractive forces within the nuclei are strong indeed. To measure the energy needed for the removal of nucleons from nuclei (the binding energy of these nucleons), a suitable unit of energy is needed.
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