Proton-rich nuclei

If, before it decays, Li is struck by a prevalent " He nucleus then "B can be formed ("Li 4-" He -> "B 4- n) and this will survive longer than in a proton-rich environment ("B 4 p -> 3" He). Other neutron-rich species could also be synthesized and survive in greater numbers than would... 

Example H,0+(aq) + HS"(s) - H2S(g) + H20(1). proton emission A nuclear decay process in which a proton is emitted. In proton emission, the mass and charge numbers of the nucleus both decrease by 1. proton-rich nucleus A nucleus that has a low proportion of neutrons and lies below the band of stability. proton transfer equilibrium The equilibrium involving the transfer of a hydrogen ion between an acid and a base. 

Proton-rich nuclei can also decay by electron capture. In this process, an electron from the innermost electron shell orbitals is captured into the nucleus and transforms a proton into a neutron (and a neutrino is emitted for conservation of energy). The vacancy created by the lost electron is filled by the transition of an electron from a higher level orbital, and the energy difference between the intervening orbitals is emitted as energy in the form of an X ray. 

Two mechanisms have been proposed for the creation of the proton-rich isotopes. The first, which involves the capture of a proton by a nucleus, appears to be most effective in producing the lighter proton-rich isotopes. The reaction is a p, y reaction whose net effect is simply to add one proton to a nucleus, increasing its atomic number and mass number by 1. A possible mechanism for the synthesis of vanadium-50 might he ... 

When a radionuclide is proton rich, it decays by the emission of a positron (/ +) along with a neutrino v. In essence, a proton in the nucleus is converted to a neutron in the process. 

Electron capture (EC). A mode of decay of a proton-rich radionuclide in which an orbital electron is captured by the nucleus, accompanied by emission of a neutrino and characteristic X-rays. 

The number of neutrons in a nucleus does not affect the identity of the element but does affect its mass. Typically, the number of neutrons in a nucleus is similar to (and generally somewhat more than) the number of protons, but numbers of neutrons can vary by a few either way—hence the isotopes. Carbon, for instance, almost always has six neutrons in addition to its six protons, but isotopes with seven and eight protons are also known. The range of numbers of neutrons is greater in the south of the kingdom, as the proportion of neutrons needed to help bind these proton-rich nuclei together increases. At uranium, for instance, the 92 protons are accompanied by about 150 neutrons, with 146 being the most common value. 

SECTION 21.2 The neutron-to-proton ratio is an important factor determining nuclear stability. By comparing a nuclide s neutron-to-proton ratio with those in the band of stability, we can predict the mode of radioactive decay. In general, neutron-rich nuclei tend to emit beta particles proton-rich nuclei tend to either emit positrons or im-dergo electron capture and heavy nuclei tend to emit alpha particles. The presence of magic numbers of nucleons and an even number of protons and neutrons also help determine the stability of a nucleus. A nuclide may undergo a series of decay steps before a stable nuclide forms. This series of steps is called a radioactive series or a nuciear disintegration series. 

The building of heavier elements from hydrogen is the source of energy for stars. Such fusion reactions are exoergic only through iron, however. Yttrium and the lanthanides are products of nuclear reactions incidental to stellar evolution, e.g., explosions of supernovae. They are the products of the stepwise capture of many neutrons by nuclei of iron or by heavier nuclei already synthesized from iron. Some lanthanide isotopes are produced when successive neutrons are captured on a slow time scale. Under those conditions, each nucleus produced by the capture of a neutron, if radioactive, had time to convert the extra neutron to a proton by beta decay before the next neutron was absorbed. Other isotopes resulted from neutron capture on an incredibly rapid time scale. Parent nuclei were exposed briefly to a flux of neutrons so intense that they absorbed all the neutrons that could be contained in energetically bound states. Afterwards, a series of beta decays ensued until a stable ratio of neutrons to protons was reached. The most proton-rich (and relatively rare) lanthanide isotopes were produced by nuclear reactions that absorbed protons. Detailed descriptions of nucleosynthetic processes are given by Clayton (1968) and in the more recent literature of astrophysics. 

We can use Fig. 17.13 to predict the type of disintegration that a radioactive nuclide is likely to undergo. Nuclei that lie above the band of stability are neutron rich they have a high proportion of neutrons. These nuclei tend to decay in such a way that the final n/p ratio is closer to that found in the band of stability. For example, a l4C nucleus can reach a more stable state by ejecting a (3 particle, which reduces the n/p ratio as a result of the conversion of a neutron into a proton (Fig. 17.15) ... 

Beta (—) decay ( ). When we consider 146C, we see that the nucleus contains six protons and eight neutrons. This is somewhat "rich" in neutrons, so the nucleus is unstable. Decay takes place in a manner that decreases the number of neutrons and increases the number of protons. The type of decay that accomplishes this is the emission of a (3 particle as a neutron in the nucleus is converted into a proton. The (3 particle is simply an electron. The beta particle that is emitted is an electron that is... 

The magic numbers which impart stability to a nucleus are 2, 8, 20, 28, 50, 82 or 122. The isotope, 39K, has a magic number equal to its number of neutrons, so it is probably stable. The others have a larger neutron-to-proton ratio, making them neutron-rich nuclei, so 40K and 41K might be expected to decay by beta emission. In fact, both 39K and 41K are stable, and 40K does decay by beta emission. 

Reports on the electrophilic substitution of the azepine nucleus at positions other than at nitrogen are uncommon, and in the main involve protonation of the hydroazepine, particularly those containing an enamine or dienamine moiety. In such cases protonation occurs predictably at the electron-rich (3- (and possibly S-) positions. For example, Paquette demonstrated spectroscopically that the dienamine (91) in perchloric acid exists as the 6,7-dihydro-4//-azepinium perchlorate (92) (B-69MI51600). 

Another question we might pose to ourselves is whether the neutron and proton distributions in nuclei are the same Modern models for the nuclear potential predict the nuclear skin region to be neutron-rich. The neutron potential is predicted to extend out to larger radii than the proton potential. Extreme examples of this behavior are the halo nuclei. A halo nucleus is a very n-rich (or p-rich) nucleus (generally with low A) where the outermost nucleons are very weakly bound. The density distribution of these weakly bound outermost nucleons extends beyond the radius expected from the R °c A1 /3 rule. Examples of these nuclei are nBe, nLi, and 19C. The most well-studied case of halo nuclei is 1 Li. Here the two outermost nucleons are so weakly bound (a few hundred keV each) as to make the size of 11 Li equal to the size of a 208Pb nucleus (see Fig. 2.12). 

We can continue our survey of the lightest nuclei with A = 3. Only the combinations of two protons and one neutron, 3He, and one proton with two neutrons, 3H, are bound, while the combinations of three protons, 3Li, and three neutrons are unbound. Again we see a balance between the numbers of neutrons and protons with the extreme cases being unbound. The nuclear spins of both bound A = 3 nuclei are j indicative of a pair of nucleons plus one unpaired nucleon three unpaired nucleons would have had a total spin of. In the A = 3 system the more neutron-rich nucleus, tritium, 3H, is very slightly less stable than 3He and, it decays by (3 emission with a 12.3-y half-life. 

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