ATOMIC NUCLEUS. - The study undertaken to discover the intimate constitution of chemical atoms led to the conclusion that every atom really consists of a nucleus and a peripheral region, in which there exist electrons linked to the nucleus. The atomic model of Rutherford at the beginning of our century was modified by N. Bohr and by A. Sommerfeld on the basis of the principles of Planck and of Pauli. Bohr's quantized model of the atom succeeded in interpreting almost exactly the emission of visible, ultraviolet and infrared radiation by the hydrogen atom; and Sommerfeld's theory made possible the classification of atoms in Stoner's table which, on the basis of the distribution of the electrons revolving around the nucleus, in turn explained the recurrence of the physico-chemical properties of atoms.
The physico-chemical properties of the elements depend exclusively on the said distribution of the satellite electrons. And this distribution depends in turn on the electric charge of the nucleus, which determines the number of such electrons and the radii of their orbits. Since the electron has a negative electric charge which, in the present state of our experimental knowledge, must be regarded as indivisible, it has been assumed as the unit charge.
Atomic nuclei have positive electric charges equal, apart from the sign, to a whole number Z of elementary charges, so that a neutral atom will be completed by an equal number Z of electrons revolving around the nucleus. This number Z, called the atomic number, therefore has a threefold meaning: 1) it measures the positive charge of the nucleus in elementary units; 2) it represents the number of negative electrons revolving around the nucleus; 3) it defines the order of the element to which the atom belongs according to a classification founded on the increasing number of satellite electrons, a classification which coincides, apart from a few exceptions, with that founded on the increasing value of the atomic mass. The elementary charge, accurately measured, proves to be 1.6.10⁻¹⁰ coulomb.
The simplest atoms are those of hydrogen (H); they consist of a nucleus having a unit positive charge and a single revolving electron. Their atomic number is therefore Z = 1. The mass of this nucleus is estimated at 1.673.10⁻²¹ grams; that is, 5.98.10²¹ of them are needed to form one gram of matter. The nucleus of H also proves to be an indivisible particle: it has been called the proton. The mass of the electron proves to be considerably smaller than that of the proton; it is 0.00091.10⁻²¹ grams, that is, 1/1840 of the mass of the proton. While the proton and electron were individually known from the beginning of our century, only later, in 1932, did we arrive at the discovery of two other isolated particles: one called the positive electron or positron, which has all the charge and mass characteristics of the negative electron, apart from the sign; the other the neutron, an electrically neutral particle having a mass slightly greater than that of the proton, that is 1.675.10⁻²¹.
It is recalled that chemistry refers atomic masses to a special unit of mass = 1/16 of the mass of an atom of oxygen (O). One unit of mass is equal to 1.6603.10⁻²¹ grams. Its inverse, that is 6.02.10²¹, is called Avogadro's number and represents the number of units of mass necessary to form the mass of one gram. To each of these elementary particles, considered as small spheres, one can attribute a diameter of ca. 3.10⁻¹⁰ cm. The number of "heavy" particles (protons only or protons + neutrons) is called the mass number. Every n. can be indicated with the symbol of the element to which it belongs, and with two numbers, M above and Z below.
For example: ₁H¹, ₄Be⁰, ₆O¹⁶, ₂₅U²³⁸ indicate respectively n. (or atoms) of hydrogen, beryllium, oxygen, uranium having atomic numbers 1,4,8,92 and mass numbers 1,9,16,238. As a consequence of the triple meaning of the atomic number, the atoms (or the n.) of equal atomic number Z belong to the same element; the atomic numbers indicated above 1,4,8,92 can only belong respectively to atoms of H,Be,O,U so that, omitting them, one does not incur ambiguity; however the same atoms (or n.) may have mass numbers different from those indicated, that is one may have atoms such as ₁H², ₁H³, ₄Be¹⁰, ₆O¹⁷, ₆O¹⁸, ₂₅U²³⁶ belonging to elements chemically equal to the preceding ones, called isotopes of these, inasmuch as in Stoner's classification or in Mendelejeff's they must occupy "the same place". A single chemical symbol serves for all the isotopes of an element; the exception is hydrogen of mass number 2, also called heavy hydrogen or deuterium, for which the symbol D may also be used. The n. of D are called deuterons.
Besides natural n., starting from 1916 one began to obtain artificial n. by modifying the former by means of "bombardment"; that is, the dream of the ancient alchemists was realized, of transforming one element into another element; however the n. thus obtained either correspond to some already known natural n. and have, in that case, all its characteristics, including its possible radioactivity, or they turn out to be all radioactive, that is unstable. The artificial n. manufactured today number many hundreds and are continually increasing. The bombardment of n. was obtained for the first time by Rutherford by placing near the substance to be bombarded a substance of α radioactivity. The α particles emitted in great number have a certain probability of striking the n., a probability that decreases as Z increases, inasmuch as the repulsive force exerted on the α particle by the n. increases (Coulomb's law) as Z increases. The n. struck absorbs the α particle and generally emits a proton or a deuteron or another particle, and is transformed into another n.
The energy of particles is customarily measured in MeV (mega-electron-volt); one MeV is the energy acquired by an electron when it is accelerated by a potential difference of one million volts. The α particles emitted by radioactive substances have energies ranging from 4.23 MeV (emitted by Th²³²) to 8.76 MeV (emitted by Po²³³; for the symbols v. CHIMICA).
One can obtain n. of protons, deuterons, α, of energies much greater than those of natural α, by means of "accelerating machines" called linear and multiple accelerators (studied and perfected by Cockrott and Walton, Van de Graaf, Alvarez), cyclotrons (Lawrence), synchrocyclotrons (Mc Millan and Weksler), bevatrons or cosmotrons; the first allow the acceleration of protons, deuterons, α up to energies of tens of MeV; cyclotrons up to 100 and synchrocyclotrons up to hundreds of MeV and finally bevatrons up to thousands of MeV (1000 MeV = 1 Bev); for example, Fermi's synchrocyclotron at Chicago can give α up to 480 MeV; in this machine, the most powerful of its kind, the electromagnet alone has a mass of iron weighing 1700 tons; the bevatron under construction at Berkeley has an annular magnet 90 meters in diameter. The protons, deuterons, α accelerated to these very high energies not only succeed in reaching the n. of greatest charge, but can produce particular reactions analogous to those that give rise to the mesotrons of cosmic rays. The possibilities of nuclear reactions are finally particularly high with bombardment of neutrons. The first experiments in this direction were undertaken in 1932 by Fermi in Rome and by Joliot in Paris. Fermi's neutron source consists of a small glass vial containing a little beryllium powder and an α radioactive substance (radium, emanation, polonium, etc.). From the impact of the α against the n. of beryllium one obtains the reaction eBe⁰ + eα² = e2Li⁰ + neutron. The neutrons have a very high penetrating power, therefore they easily exit from the glass tube. In Fermi's neutron source the production of one neutron is obtained for every 30,000 α ca. Since one gram of radium emits 37 billion α per second, from them one obtains ca. one million neutrons per second.
The atomic piles devised and built for the first time by Fermi represent sources of neutrons enormously more intense than the Ra + Be preparations. One of the most modest atomic piles, the French one called ZOE, can provide a flux of ca. 10¹¹ neutrons per second, equal that is to that obtainable from a Ra + Be source containing a quintal of Ra. The artificial radioactive n. obtained with bombardments of various kinds have disintegration characteristics similar to those of natural n. But besides these phenomena there exists one of particular importance, discovered in 1939 by Hahn. Uranium α 1²³⁵ struck by a neutron breaks into two almost equal parts and into a certain number of neutrons (from 0 to 6, on average 2.4).
The phenomenon has been called fission (Eng. fission); it is presented besides by U²³⁵ by some other heavy elements, above all by Pu²³⁶, for neutrons of every energy. U²³⁶ instead splits only if the neutron that strikes it has an energy greater than 2 MeV. The atomic pile is a device in which there is a controlled succession of fissions that continues "in chain" thanks to the neutrons that are generated in the fissions and that in turn can produce other fissions; this chain reaction cannot occur with U²³⁸ because the neutrons liberated in the fission have energies lower than 2 MeV. The nuclear reaction of the atomic bomb is also of the same nature; while in the pile the number of successive fissions is controlled, in the bomb one arranges that the successive generations of neutrons increase as rapidly as possible, so as to obtain a reaction of explosive character.
The masses of the nuclei have been accurately measured by means of an instrument devised at Cambridge by Aston and called a mass spectrograph. The results of the measurements have shown that every single nucleus has a mass slightly less than what one should expect as a consequence of the number of protons and neutrons that compose it. Every nucleus thus presents a "mass defect" Δ m, as if its elementary components, when they are "packed," underwent a decrease in their mass. This decrease has been considered as the equivalent of the energy necessary to bring the nucleons (= protons + neutrons) together and pack or bind them (in German this energy is called packungenergie; in English binding energy) and it proves always to increase with atomic mass, except for some irregularities found in the first elements. On the basis of Einstein's relation: E = c²Δ m; if E is measured in MeV and Δ m in units of mass, the constant c² takes the value 931 MeV/unit of mass (which still has the dimension of the square of a velocity). If one calculates the mass defect of the individual nucleons that constitute the nucleus, that is, the value of Δ M/M, one finds that it varies with the mass number M, as is indicated in the figure.
With every nuclear transformation there is a variation of the mass defect; this produces a release of energy (exothermic reaction) if it transforms one nucleus into another with a greater mass defect, which happens when one passes from nuclei with a very large or very small M toward nuclei with an intermediate M. On the contrary, if one starts from nuclei with an intermediate M toward nuclei with a greater or smaller M, there is always a decrease of the mass defect, and in this case the reaction is always endothermic, that is, with absorption rather than with release of energy. The graph in the figure makes it possible to know whether a given nuclear reaction is or is not exothermic and what energy is released or absorbed in it.
Bibliography6 works
Tito Franzini