ATOMIC NUCLEUS. — The study to discover the intimate constitution of chemical atoms led to the conclusion that every atom actually consists of an atomic nucleus and a peripheral region in which electrons exist, bound to the nucleus. The atomic model from Rutherford at the beginning of our century was modified by N. Bohr and A. Sommerfeld on the basis of Planck’s and Pauli’s principles. Bohr’s quantized atom model succeeded in interpreting almost exactly the emission of visible, ultraviolet, and infrared radiations by the hydrogen atom; and Sommerfeld’s theory allowed the classification of atoms in Stoner’s table, which, based on the distribution of electrons rotating around the nucleus, in turn explained the recurrence of the physical-chemical properties of atoms.
The physical-chemical properties of elements depend exclusively on the distribution of the satellite electrons. And this distribution in turn depends 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, according to our present experimental knowledge, must be considered indivisible, it has been taken as the unit charge.
Atomic nuclei have positive electric charges equal, apart from the sign, to an integer Z of elementary charges, so that a neutral atom will be completed with an equal number Z of electrons rotating 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 rotating around the nucleus; 3) it defines the order of the element to which the atom belongs according to a classification based on the increasing number of satellite electrons, a classification which, apart from a few exceptions, coincides with that based on the increasing value of atomic mass. The elementary charge, accurately measured, is 1.6 × 10⁻¹⁰ coulomb.
The simplest atoms are those of hydrogen (H); they consist of a nucleus with a unit positive charge and a single rotating electron. For these, therefore, the atomic number Z = 1. The mass of such a nucleus is estimated at 1.673 × 10⁻²⁴ grams; that is, 5.98 × 10²² nuclei are needed to form one gram of matter. Even the H nucleus turns out to be an indivisible particle: it has been called the proton. The mass of the electron is found to be considerably smaller than that of the proton; it is 0.0009 × 10⁻²³ grams, or 1/1840 the mass of the proton. While proton and electron were individually known since the beginning of our century, only later, in 1932, was the discovery made 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 with a mass slightly greater than that of the proton, namely 1.675 × 10⁻²⁴ grams.
It should be noted that chemistry refers atomic masses to a special unit of mass equal to 1/16 the mass of an oxygen atom (O). One unit of mass is equal to 1.6603 × 10⁻²⁴ grams. Its inverse, namely 6.02 × 10²³, is called Avogadro’s number and represents the number of mass units needed to form the mass of one gram. To each of these elementary particles, considered as small spheres, a diameter of about 3 × 10⁻¹³ cm can be attributed. The number of “heavy” particles (protons alone or protons + neutrons) is called the mass number. Every nucleus can be indicated with the symbol of the element to which it belongs, and with two numbers, M above and Z below.
E.g.: ¹H₁, ⁴Be₉, ⁸O₁₆, ⁹₂U₂₃₈ indicate respectively nuclei (or atoms) of hydrogen, beryllium, oxygen, uranium having atomic numbers 1, 4, 8, 92 and mass numbers 1, 9, 16, 238. In consequence of the threefold meaning of the atomic number, atoms (or nuclei) with the same atomic number Z belong to the same element; the atomic numbers given above, 1, 4, 8, 92, can only belong respectively to atoms of H, Be, O, U so that, omitting them, no ambiguity arises; however, the same atoms (or nuclei) can have different mass numbers from those indicated, that is, atoms such as ¹H₂, ¹H₃, ⁴Be₁₀, ⁸O₁₆, ⁹₂U₂₃₈ can exist, chemically equal to the previous ones, called isotopes of these, since in Stoner’s or Mendeleev’s classification they must occupy “the same place.” A single chemical symbol serves for all the isotopes of an element; the exception is hydrogen with mass number 2, also called heavy hydrogen or deuterium, for which the symbol D can also be used. The nuclei of D are called deuterons.
Besides natural nuclei, starting from 1916 artificial nuclei have been obtained by modifying the former through “bombardment”; that is, the dream of the alchemists has been realized of transforming one element into another; however, those obtained either correspond to some already known natural nucleus and in that case have all its characteristics, including any radioactivity, or they are all radioactive, i.e., unstable. The artificial nuclei manufactured today number in the hundreds and are continually increasing. The bombardment of nuclei was first achieved by Rutherford by placing a radioactive substance near the substance to be bombarded. The α-particles emitted in large numbers have a certain probability of striking the nuclei, a probability that decreases as Z increases, since the repulsive force exerted on the α-particle by the nucleus increases (Coulomb’s law) with increasing Z. The nucleus struck absorbs the α-particle and generally emits a proton or a deuteron or another particle, and is transformed into another nucleus.
The energy of the particles is usually measured in MeV (million-electron-volts); one MeV is the energy acquired by an electron when 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 Po²¹²); for the symbols V. chemistry.
Protons, deuterons, and electrons of energies far greater than those of natural particles can be obtained by means of certain accelerating machines called linear and multiple accelerators (studied and perfected by Cockcroft and Walton, Van de Graaf, Alvarez), cyclotrons (Lawrence), synchrocyclotrons (McMillan and Weeks), bevatrons or cosmotrons; the first allow the acceleration of protons, deuterons, and electrons up to energies of tens of MeV; cyclotrons reach 100 MeV, synchrocyclotrons hundreds of MeV, and finally bevatrons thousands of MeV (1000 MeV = 1 BeV); e.g., the synchrocyclotron of Fermi in Chicago can give electrons up to 480 MeV; in this machine, the most powerful of its kind, the sole electromagnet has an iron mass weighing 1700 tons; the bevatron under construction at Berkeley has an annular magnet of 90 meters in diameter. The protons, deuterons, and electrons accelerated to these very high energies not only succeed in reaching the nuclei of higher 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 neutron bombardment. The first experiments in this sense were undertaken in 1932 by Fermi in Rome and by Joliot in Paris. Fermi’s neutron source consists of a small glass tube containing a little beryllium powder and a radioactive α-emitter (radium, radon, polonium, etc.). From the impact of the α-particles against the beryllium nuclei, the reaction \(^{7}_{3}Be^{4} + ^{1}_{0}n \rightarrow ^{4}_{2}Li^{0} + \text{neutron}\) is obtained. Neutrons have a very high penetrating power, so they easily escape from the glass tube. In Fermi’s neutron source, the production of one neutron is obtained for every 30,000 α-particles. Since one gram of radium emits 37 billion α-particles per second, about a million neutrons per second are obtained from II.
The atomic piles devised and constructed for the first time by Fermi represent neutron sources far more intense than Ra + Be preparations. One of the more modest atomic piles, the French one called ZOE, can provide a flux of about \(10^{11}\) neutrons per second, equal to that obtainable from an Ra + Be source containing a hundredweight of radium. The artificial radioactive nuclei obtained by bombardments of various kinds have decay characteristics similar to those of natural nuclei. But besides these phenomena, there exists one of particular importance, discovered in 1939 by Hahn. Uranium \(^{238}_{92}U^{235}\) struck by a neutron splits into two nearly equal parts and a certain number of neutrons (from 0 to 6, on average 2.4).
The phenomenon has been called fission (Eng. fission); it is presented not only by U\(^{235}\) but also by some other heavy elements, especially by Pu\(^{239}\), for neutrons of any energy. U\(^{238}\) splits instead only if the neutron that strikes it has an energy greater than 2 MeV. The atomic pile is a device in which a controlled succession of fissions takes place in a chain reaction, thanks to the neutrons generated in the fissions, which in turn can produce other fissions; this chain reaction cannot occur with U\(^{238}\) because the neutrons freed in the fission have energies less than 2 MeV. The nuclear reaction of the atomic bomb is of the same nature; while in the pile the number of successive fissions is controlled, in the bomb an effort is made to ensure that the successive generations of neutrons increase as rapidly as possible, so as to obtain a reaction of an explosive character.
The masses of nuclei have been accurately measured by means of an instrument devised at Cambridge by Aston and called the mass spectrograph. The results of the measurements have shown that each individual nucleus has a mass slightly less than that which one would expect from the number of protons and neutrons composing II. Each nucleus thus presents a mass defect Δm, as if the elementary components, when unbound, underwent a decrease in their mass. This decrease has been considered as the equivalent of the energy E necessary to bring the nucleons (p = protons + neutrons) from the unbound or free state to the bound state (in German this energy is called Packungsenergie; in English, binding energy), and it always increases with atomic mass, apart from certain irregularities found in the first elements. On the basis of Einstein’s relation: \(E = c^2 \Delta m\), if E is measured in MeV and Δm in units of mass, the constant \(c^2\) assumes the value 931 MeV/unit of mass (which still has the dimension of a velocity squared). If one calculates the mass defect of the individual nucleons that constitute the nucleus, i.e., the value of Δm/M, one finds that it varies with the mass number M, as is shown in the figure.
In every nuclear transformation there is a variation of the mass defect; this produces a development of energy (exothermic reaction) if it transforms a nucleus into another with a greater mass defect, which occurs when one passes from nuclei with M very large or very small to nuclei with intermediate M. On the contrary, if one starts from nuclei with intermediate M toward nuclei with greater or smaller M, there is always a decrease of the mass defect, and in this case the reaction is always endothermic, i.e., with absorption rather than development of energy. The graph in the figure allows one to know whether a given nuclear reaction is exothermic or not and what energy is developed or absorbed in II.