The Stability of Atomic Nuclei

Write an equation showing the emission of a beta particle from jPa. 

Those isotopes with too few neutrons (red dots below the band of stability) decay as well, but in a manner that increases the number of neutrons relative to the number of protons. One way this can happen is by emission of a type of subatomic particle discovered in 1932, a positron— a positively chained electron, e. For example, the decay of nitrogen-13, an isotope with too few neutrons, is by positron emission. 

Perhaps because there are so many unstable (radioactive) isotopes, people sometimes think of radioactive when the word isotope is mentioned. 

Beta particles result from the conversion of neutrons into protons and electrons 

The positron is sometimes called the antielectron. The positron is one of a group of antimatter particles known to exist. An electron will react with a positron to annihilate each other and produce two high-energy gamma rays. 

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The ratio of the number of neutrons to protons determines the stability of atomic nuclei. As the number of protons in the nucleus increases, the number of neutrons must increase at a greater rate to be stable. 

One may rightfully raise the question as to why some products of nuclear reactions are radioactive while others are not. The answer concerns the stability of atomic nuclei. Essentially, any radioactive element, whether artificial or natural, can be considered abnormal. A nucleus that undergoes radioactive decay is in an unstable condition, and the process of decay always leads to stable isotopes. This tendency toward the achievement of stability is illustrated by the stepwise decay of naturally radioactive uranium to form a stable isotope of lead and the formation of stable carbon by the decay of artificial radioactive nitrogen. Although the conditions resulting in the instability of atomic nuclei are fairly well understood, further consideration of these factors is beyond the scope of this discussion. 

This mass-energy relation must dominate the whole question of the genesis of elements, the dynamics of stars, the origin of cosmic radiations, and other fundamental matters. The only respect in which it affects ordinary terrestrial chemistry is in connexion with the stability of atomic nuclei. The changes which occur in chemical reactions are too small to affect practically the traditional principle of mass conservation upon which so much of chemical theory has been based. 

The stability of atoms - their property of being steadfast and remaining unchanged - is determined by the nature of their nuclei (see Textbox 12). Nuclei in which the number of neutrons is smaller than or equal to the number of protons are stable, while those in which the number of neutrons is larger than the number of protons are unstable. Unstable nuclei have a tendency to adjust the disparity between the number of neutrons and protons and become stable they may do so by one of two processes, by radioactive decay or nuclear fission. 

Isotopes can differ significantly in one respect the stability of their nuclei. The nucleus of a carbon atom, for instance, will happily... 

W. Harkins, The constitution and stability of atom nuclei, Philosophical Magazine 42 (1921) 305-339, on 310. See also W. Harkins, Isotopes Their number and classification," Nature 107 (1921) 202-203, which includes what is probably the first diagram of the abundance of isotopes as a function of the atomic number. Like all other physicists at the time, Harkins believed that atomic nuclei consisted of protons and electrons. The number of electrons corresponds to the quantity A-Z, later identified with the neutron number. 

Two factors affect the stability of this orbital. The first is the stabilizing influence of the positively charged nuclei at the center of the AOs. This factor requires that the center of the AO be as close as possible to the nucleus. The other factor is the stabilizing overlap between the two constituent AOs, which requires that they approach each other as closely as possible. The best compromise is probably to shift the center of each AO slightly away from its own nucleus towards the other atom, as shown in figure 7-23a. However, these slightly shifted positions are only correct for this particular MO. Others may require a slight shift in the opposite direction. 

In graphite each carbon atom is bound to three others in the same plane and here the assumption of inversion of a puckered layer is improbable, because of the number of atoms involved. A probable structure is one in which each carbon atom forms two single bonds and one double bond with other atoms. These three bonds should lie in a plane, with angles 109°28 and 125°16,l which are not far from 120°. Two single bonds and a double bond should be nearly as stable as four single bonds (in diamond), and the stability would be increased by the resonance terms arising from the shift of the double bond from one atom to another. But this problem and the closely related problem of the structure of aromatic nuclei demand a detailed discussion, perhaps along the lines indicated, before they can be considered to be solved. 

The hetero radicals that have already been referred to—(9, p. 301), (10, p. 302), (14, p. 302) and (15, p. 302)—owe their relative stability [with respect to their dimers—apart from l,l-diphenyl-2-picrylhydrazyl (10)] to a variety of factors (a) the relative weakness of N—N, S—S and 0—0 bonds, (b) the delocalisation through the agency of aromatic nuclei, and (c) steric inhibition of access to the atom with the unpaired electron, or to an aryl p-position, cf. (50). The latter factor bulks large (in addition to the weakness of O—O bonds) in the great stability of (15, cf. p. 302) and all three factors operate to stabilise (51), which is wholly dissociated in solution ... 

The stability of nuclides is characterized by several important rules, two of which are briefly discussed here. The first is the so-called synunetry rule, which states that in a stable nuclide with low atomic number, the number of protons is approximately equal to the number of neutrons, or the neutron-to-proton ratio, N/Z, is approximately equal to unity. In stable nuclei with more than 20 protons or neutrons, the N/Z ratio is always greater than unity, with a maximum value of about 1.5 for the heaviest stable nuclei. The electrostatic Coulomb repulsion of the positively charged protons grows rapidly with increasing Z. To maintain the stability in the nuclei, more... 

J Ju elements in the periodic table exist in unstable versions called radioisotopes (see Chapter 3 for details). These radioisotopes decay into other (usually more stable) elements in a process called radioactive decay. Because the stability of these radioisotopes depends on the composition of their nuclei, radioactivity is considered a form of nuclear chemistry. Unsurprisingly, nuclear chemistry deals with nuclei and nuclear processes. Nuclear fusion, which fuels the sun, and nuclear fission, which fuels a nuclear bomb, are examples of nuclear chemistry because they deal with the joining or splitting of atomic nuclei. In this chapter, you find out about nuclear decay, rates of decay called half-lives, and the processes of fusion and fission. 

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