Neutron isotopes

Isotopes of elements are identified by their mass numbers. Hence, the isotope C-14 is the form of carbon that contains eight neutrons. Isotopes of an element have similar chemical properties. Therefore, when we eat a bowl of cereal and incorporate carbon in our bodies, we are assimilating C-14 (and other carbon isotopes) along with the common form of carbon, C-12. 

Once the primary isotopes had built up in abundance, then the full range of reactions discussed above became available and nuclides such as C, N, O, O, F, Ne, Ne, 25Mg, 26Mg, and so on, were produced by reactions between primary isotopes and protons and neutrons. Isotopes that require the presence of metals in the initial composition of the star in order to be efficiently synthesized are secondary isotopes. Abundances of primary isotopes built up rapidly in the early universe via synthesis in massive stars. Secondary isotopes initially built up more slowly, but their rate of synthesis increased as metallicity increased. 

All odd-Z elements lighter than jluorine have a stable isotope having equal numbers 0/ neutrons and protons (N = Z). But no odd-Z elements heavier than jluorine have a stable isotope having N = Z. Fluorine lies on this curious border, whose explanation involves the repulsive electric/orce. The energy 0/repulsion o/Z protons to themselves is proportional to Z2, and/or Z > g that repulsive energy is so great that the odd-Z nucleus is radioactively unstable to beta decay to the even-Z element 0/the same mass number. The nine-neutron isotope 0/ F, l8F, decays to l80, /or example. 

Isotopes are atoms of the same element with the same number of protons (same atomic number) but different number mass numbers (due to a different number of neutrons). Isotopes have identical chemical properties (the same reactivity) but different physical properties (i.e., some are radioactive, while others are stable). Consider the three isotopes of hydrogen in Table 2.1. 

Isotope (Section 1.1) Two or more atoms of the same element having the same number of protons in the nucleus but a different number of neutrons. Isotopes have the same atomic number but different mass numbers. 

The structural information at an atomic level is essential for understanding the various properties of supercooled and glassy solutions. X-ray and neutron diffraction enables us to obtain direct structure information (bond distance and coordination number) of ionic solutions in terms of the radial distribution function. In the case of aqueous lithium halide solutions. X-ray diffraction data are dominated by halide-oxygen, halide-oxygen, and oxygen-oxygen interactions. On the contrary, neutron isotopic substitution... 

ND, neutron isotopic difference diffraction XD, X-ray isomorphic difference diffraction TX, total X-ray diffraction TN, total neutron diffraction EX, EXAFS (extended X-ray absorption fine structure). 

The complex cation ND4+ has been studied hy X-ray diffraction (49) and by the neutron isotopic difference method (31a), and results show that the coordination is weak, as might be expected for the low charge density. An MD computer simulation study using modified L-J potentials for the ion-water interactions gave good agreement with the ND results (75). 

As you might expect, the isotopes do differ in mass. Isotopes containing more neutrons have a greater mass. In spite of differences in mass and the number of neutrons, isotopes of an atom have essentially the same chemical behavior. Why Because, as you ll learn in greater detail later in this textbook, chemical behavior is determined by the number of electrons an atom has, not by its number of neutrons and protons. To make it easy to identify each of the various isotopes of an element, chemists add a number after the element s name. The number that is added is called the mass number, and it represents the sum of the number of protons and neutrons in the nucleus. For example, the potassium isotope with 19 protons and 20 neutrons has a mass number of 39 (19 + 20 = 39), and the isotope is called potassium-39. The potassium isotope with 19 protons and 21 neutrons has a mass number of 40 (19 -I- 21 = 40), and is called potassium-40. What is the mass number and name of the potassium isotope with 19 protons and 22 neutrons ... 

As you may recall, isotopes are atoms of the same element that have different numbers of neutrons. Isotopes of atoms with unstable nuclei are called radioisotopes. These unstable nuclei emit radiation to attain more stable atomic configurations in a process called radioactive decay. During radioactive decay, unstable atoms lose energy by emitting one of several types of radiation. The three most common types of radiation are alpha (a), beta ((3), and gamma (7). Table 25-2 summarizes some of their important properties. Later in this chapter you ll learn about other types of radiation that may be emitted in a nuclear reaction. 

A nuclear species (nuclide) is characterized by its atomic number Z (that is, the nuclear charge in units of e, or the number of protons in the nucleus) and its mass number A (the sum of the number of protons plus the number of neutrons in the nucleus). We denote an atom that contains such a nuclide with the symbol zX, where X is the chemical symbol for the element. The atomic number Z is sometimes omitted because it is implied by the chemical symbol for the element. Thus, JH (or H) is a hydrogen atom and (or C) is a carbon atom with a nucleus that contains six protons and six neutrons. Isotopes are nuclides of the same chemical species (that is, they have the same Z), but with different mass numbers A, and therefore different numbers of neutrons in the nucleus. The nuclear species of hydrogen, deuterium, and tritium, represented by JH, jH, and jH, respectively, are all members of the family of isotopes that belong to the element hydrogen. 

Matter consists of atoms that are made up of protons (electropositive), electrons (electronegative), and neutrons (electrically neutral). Because the number of electrons and protons is equal, elements, atoms with different numbers of protons, have different numbers of electrons. The chemical properties of an element depend on the number of electrons, but because the electrons have almost no mass, the atomic weight of an element is its number of protons and neutrons. Neutrons are needed to hold the protons together in the nucleus. Isotopes are elements with different numbers of neutrons. Isotopes with too many neutrons are unstable and emit radioactivity. Radioactive and nonradioactive isotopes are used to follow biochemical reactions in health and disease, to date paleontology specimens, usually bones and teeth, and detect traces of life in ancient rocks. 

The definition of an element became more precise at the dawn of the 20th century with the discovery of the proton. We now know that an atom has a small center called the nucleus. In the nucleus are one or more protons, positively charged particles, the number of which determine an atom s identity. The number of protons an atom has is referred to as its atomic number. Hydrogen, the lightest element, has an atomic number of 1, which means each of its atoms contains a single proton. The next element, helium, has an atomic number of 2, which means each of its atoms contain two protons. Lithium has an atomic number of 3, so its atoms have three protons, and so forth, all the way through the periodic table. Atomic nuclei also contain neutrons, but atoms of the same element can have different numbers of neutrons we call atoms of the same element with different number of neutrons isotopes. ... 

Each nucleus is characterized by a definite atomic number Z and mass number A for clarity, we use the symbol M to denote the atomic mass in kinematic equations. The atomic number Z is the number of protons, and hence the number of electrons, in the neutral atom it reflects the atomic properties of the atom. The mass number gives the number of nucleons (protons and neutrons) isotopes are nuclei (often called nuclides) with the same Z and different A. The current practice is to represent each nucleus by the chemical name with the mass number as a superscript, e.g., 12C. The chemical atomic weight (or atomic mass) of elements as listed in the periodic table gives the average mass, i.e., the average of the stable isotopes weighted by their abundance. Carbon, for example, has an atomic weight of 12.011, which reflects the 1.1% abundance of 13C. 

Electrons Nuclear atom Nucleus Proton Neutron Isotopes Atomic number Mass number... 

Recall that isotopes are atoms of the same element that have different numbers of neutrons. Isotopes of atoms with unstable nuclei are called radioisotopes. These unstable nuclei emit radiation to attain more stable atomic configurations in a process called radioactive decay. During radioactive decay, unstable atoms lose energy by emitting radiation. 

All elements are represented by several isotopes. With the same number of protons they have different number of neutrons. Isotopes of the same element have identical electron shells and are almost indistinguishable in their chemical properties. They differ mostly in the mass. The hghter the element, the greater this difference. 

Gold has 30 known isotopes, but only one, Au, is stable. The nucleus of Au contains 79 protons and 118 neutrons. Isotopes of mass numbers 177-183 are all a emitters and all have a physical half-life of < 1 minute. Isotopes of mass numbers 185-196 decay by electron capture accompanied by y radiation and in some cases by positron emission. The only long-lived isotope is Au with a half-life of 183 days. The neutron-heavy isotopes of 198-204 all decay by b emission accompanied by y radiation. The isotope Au is widely used in radiotherapy, in medical diagnosis, and for tracer studies. 

In any reactor where the reaction composition is circulated outside of the reaction zone, delayed neutrons are lost to the reaction if they are released while outside of the reactor. About 1 percent of the neutrons emitted as a result of fission are delayed. Since all of the delayed emission periods, except the first one, are long compared with the uninterrupted sojourn of the within the reactor, the percentage of delayed neutrons emitted outside the reactor will be equal to the neutron Isotope holdup outside the reactor divided by the total amount of reaction isotope. 

Element 61, promethium, has no stable isotopes.) The elements of even Z (Ce, Nd, Sm, Gd, Dy, Er, Yb) are largely composed of even-even isotopes with 1=0, together with (apart from Ce) odd-neutron isotopes of 10 to 20% abundance. For these, optical spectroscopy has in general provided insufficient resolution for reliable determination even of the values of the nuclear spins. 

As mentioned in section 1.2, the hyperfine structure is much better resolved in EPR than in optical spectra the first example, the spectra of the two Nd isotopes of mass 143 and 145, is shown in fig. 2. Those of odd-neutron isotopes of other trivalent ions of Sm, Dy, Er and Yb followed soon afterwards, together with the odd-proton nuclei of Pr, Tb, Ho and Tm. Calculation of the nuclear moments from the hyperfine structure depended on estimates of the value of these have... 

Detailed ENDOR measurements have been made by Chan and Hutchison (1972) on LaClj doped with samarium, oihanced in the two odd-neutron isotopes 147,149 (both 1 = j). The energies of the two low-lying doublet levels of J = f, split by the crystal field, as well as the splitting of the J = 1 manifold, have been determined by optical spectroscopy (Varsanyi and Dieke 1961, Magno and Dieke 1962). Also, the size of the 7-mixing (Axe and Dieke 1962) was evaluated from these measurements. The ENDOR experiments determine the net hyperfine interactions in the lowest doublet, together with the effective nuclear Zeeman interaction, the tensor... 

Deuterium refers to the mass-two (one proton and one neutron) isotope of hydrogen and is symbolized by or D. There is also a radioactive mass-three (one proton and two neutrons) isotope called tritium and symbolized by or T. 

II. Alpha particles and atomic nuclei are positively charged. As an alpha particle approached a nucleus, the repulsion between the positive charges deflected the alpha particle from its path. 13. The nucleus. 15. The electrons were thought to travel in circular orbits around the nucleus. 17. No, the atomic number is the number of protons, and all atoms of an element have the same number of protons. 19. Mass nmnber is the sum of protons plus neutrons. Isotopes of the same element have different numbers of neutrons, but the same number of protons. The sirnis must be different. Atoms of different elements must have different numbers of protons and they may have different numbers of neutrons. An atom with one less proton than another may have one more neutron than the other, so their mass numbers would be the same. Example carbon-14 (6 protons, 8 neutrons) and nitrogen-14 (7 protons, 7 neutrons). 

Atoms with the same number of protons but with a different number of neutrons are referred to as isotopes, with the mass of the specific atom defined by the sum of the protons and neutrons. For example. Carbon 12 (mass equals 12 u) has six protons and six neutrons. Its chemical symbol is C. Carbon 13 (( C)), on the other hand, has six protons and seven neutrons. Isotopic mass is covered in Section 2.1.1.1. 

Sources of Neutrons.—Isotopic Neutron Sources. Conventional isotopic sources of neutrons utilize the production of neutrons by the (a,n) reaction (e.g, Am-Be) or the (y,n) reaction (e.g. Sb-Be). Typical sources give a neutron output of 10 —10 s Ci S depending upon the composition the usable neutron fluxes are of the order of 10 —10 n cm" s" The neutron energies are of the order of 1—10 MeV energy for (a,n) sources (26 keV for a Sb-Be source) and in many cases neutron moderation will be necessary, reducing the usable neutron flux. The low fluxes available preclude their use... 

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