Stable nuclides

Natural abundance. The natural abundances listed are on an atom percent basis for the stable nuclides present in naturally occurring elements in the earth s crust. 

It will be recalled that is 100% abundant and is the heaviest stable nuclide of any element (p. 550), but it is essential to use very high purity Bi to prevent unwanted nuclear side-reactions which would contaminate the product Po in particular Sc, Ag, As, Sb and Te must be <0.1 ppm and Fe <10ppm. Polonium can be obtained directly in milligram amounts by fractional vacuum distillation from the metallic bismuth. Alternatively, it can be deposited spontaneously by electrochemical replacement onto the surface of a less electropositive metal... 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

Figure 17.13 is a plot of mass number against atomic number for known nuclides. Stable nuclei are found in a band of stability surrounded by a sea of instability, the region of unstable nuclides that decay with the emission of radiation. For atomic numbers up to about 20, the stable nuclides have approximately equal numbers of neutrons and protons, and so A is close to 2Z. For higher atomic numbers, all known nuclides—both stable and unstable—have more neutrons than protons, and so A > 2Z. 

FIGURE 17.12 The numbers of stable nuclides having even or odd numbers of neutrons and protons. With the exception of hydrogen, by far the greatest number of stable nuclides (157) have even numbers of both protons and neutrons. Only four stable nuclides have odd numbers of both protons and neutrons. 

Eigure 22-2 illustrates this process schematically for fluorine 9p + 10iH F Because any stable nucleus is more stable than its separated nucleons, nuclear formation reactions of all stable nuclides are exothermic. 

The energy change is negative, indicating that this helium nuclide is more stable than its separate component particles. We expect this for any stable nuclide. 

Plot of the binding energy per nucleon vs. mass number A. The most stable nuclides lie in the region around... 

As described in Chapter 2 (see Figure ), stable nuclides fall within a belt of stability with roughly equal numbers of neutrons and protons. Lighter nuclides lie along the = Z line, but as the mass of the nuclide increases, the... 

Within the belt of stability. Table 22-2 shows that nuclides with even numbers of protons and neutrons are more prevalent than those with odd numbers of protons or neutrons. Almost 60% of all stable nuclides have even numbers of both protons and neutrons, whereas fewer than 2% have odd numbers of both. Moreover, of the five... 

Unstable nuclides decompose spontaneously Into other, more stable nuclides. These decompositions are called nuclear decay, and unstable nuclides are called radioactive. Three features characterize nuclear decays the... 

Nuclides that are unstable because they have odd-odd composition can be converted into stable nuclides with even -even composition through any of three processes electron emission, positron emission, or electron capture. Each... 

Radon-222 is an unstable nuclide that has been detected in the air of some homes. Its presence is a concern because of high health hazards associated with exposure to its radioactivity. Gaseous radon easily enters the lungs, and once it decays, the products are solids that remain embedded in lung tissue. Radon-222 transmutes to a stable nuclide by emitting a and P particles. The first four steps are a, a, P, p. Write this sequence of nuclear reactions and identify each product. 

Eission products often are radioactive. This is because the fissioning nucleus has an // Z ratio of 1.54, so its products have a similar N Z ratio, hi contrast, stable nuclides in the = 77 to 157 range have ratios of around 1.3, so the products of fission have excess neutrons, making them unstable. 

Neutron capture and P emission forms nuclei of ever higher atomic number. Neutron capture and P emission by Co (Z = 27) produces Ni (Z = 28), Ni produces Cu (Z = 29), and so on up the atomic-number ladder Neutron capture and P emission form all possible stable nuclides during the lifetime of a second-generation star. 

C22-0040. Use atomic masses to compute the total binding energy and the binding energy per nucleon for elemental cesium, which has just one stable nuclide. 

Progeny—The decay product or products resulting after a radioactive decay or a series of radioactive decays. The progeny can also be radioactive, and the chain continues until a stable nuclide is formed. 

The numerical combination of protons and neutrons in most nuclides is such that the nucleus is quantum mechanically stable and the atom is said to be stable, i.e., not radioactive however, if there are too few or too many neutrons, the nucleus is unstable and the atom is said to be radioactive. Unstable nuclides undergo radioactive transformation, a process in which a neutron or proton converts into the other and a beta particle is emitted, or else an alpha particle is emitted. Each type of decay is typically accompanied by the emission of gamma rays. These unstable atoms are called radionuclides their emissions are called ionizing radiation and the whole property is called radioactivity. Transformation or decay results in the formation of new nuclides some of which may themselves be radionuclides, while others are stable nuclides. This series of transformations is called the decay chain of the radionuclide. The first radionuclide in the chain is called the parent the subsequent products of the transformation are called progeny, daughters, or decay products. 

Table 2.2 lists the most important syntheses occurring in the stars. The main products include the bioelements C, O, N and S. The synthesis of the elements began in the initial phase after the big bang, with that of the proton and the helium nucleus. These continue to be formed in the further development of the stars. The stable nuclide 4He was the starting material for subsequent nuclear syntheses. Carbon-12 can be formed in a triple a-process, i.e., one in which three helium... 

Table 1.3 Numbers of Stable Nuclides Having Different Arrangements of Nucleons. ...

Scientists have known that nuclides which have certain "magic numbers" of protons and neutrons are especially stable. Nuclides with a number of protons or a number of neutrons or a sum of the two equal to 2, 8, 20, 28, 50, 82 or 126 have unusual stability. Examples of this are He, gO, 2oCa, Sr, and 2gfPb. This suggests a shell (energy level) model for the nucleus similar to the shell model of electron configurations. 

F has 9 protons and 8 neutrons. Replacing a proton with a neutron would produce a more stable nuclide. Thus, we predict positron emission by 17 F to produce 17 O. 

Using nuclide-stability rules, form a hypothesis that explains why calcium-40 should be a more stable nuclide than potassium-40. 

Astatine - the atomic number is 85 and the chemical symbol is At. The name derives from the Greek astatos for unstable since it is an unstable element. It was first thought to have been discovered in nature in 1931 and was named alabamine. When it was determined that there are no stable nuclides of this element in nature, that claim was discarded. It was later shown that astatine had been synthesized by the physicists Dale R. Corson, K. R. Mackenzie and Emilio Segre at the University of California lab in Berkeley, California in 1940 who bombarded bismuth with alpha particles, in the reaction Bi ( He, 2n ) "At. Independently, a claim about finding some x-ray lines of astatine was the basis for claiming discovery of an element helvetium, which was made in Bern, Switzerland. However, the very short half-life precluded any chemical separation and identification. The longest half-life associated with this unstable element is 8.1 hour °At. 

All stable nuchdes fall above the N = Z line, with the exception of H and He. Up to jN, the number of neutrons does not exceed the number of protons by more than 1. Above the stable nuclides diverge from the N/Z = 1 line upward, as a result of a progressive increase in the proportion of neutrons. 

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