Nucleon A particle in an atomic nucleus

Nucleon a particle in an atomic nucleus, either a neutron or a proton. (2.6)... 

The observation that atoms of a single element can have different masses helped scientists refine the nuclear model still further. They realized that an atomic nucleus must contain subatomic particles other than protons and proposed that it also contains electrically neutral particles called neutrons (denoted n). Because neutrons have no electric charge, their presence does not affect the nuclear charge or the number of electrons in the atom. However, they do add substantially to the mass of the nucleus, so different numbers of neutrons in a nucleus give rise to atoms of different masses, even though the atoms belong to the same element. As we can see from Table B.l, neutrons and protons are very similar apart from their charge they are jointly known as nucleons. 

The periodic table provides the chemist with a very brief and simple representation of the elements. Recall that most of the mass of an individual atom is contained in the nucleus. In addition, a nucleus contains one or more protons and, with the exception of ordinary hydrogen (1H), one or more neutrons. As discussed in chapter 2, the mass number (A) is an integer and is equal to the number of protons (Z) plus the number of neutrons (N) in the nucleus. It is the number of protons that determines what element a particular atom is. The general term nucleon is used to describe both neutrons and protons, or particles found in the atomic nucleus. 

Physicists and chemists have developed various perturbation-theory methods to deal with systems of many interacting particles (nucleons in a nucleus, atoms in a solid, electrons in an atom or molecule), and these methods constitute many-body perturbation theory (MBPT). In 1934, Mpller and Plesset proposed a perturbation treatment of atoms and molecules in which the unperturbed wave function is the Hartree-Fock function, and this form of MBPT is called Moller-Plesset (MP) perturbation theory. Actual molecular applications of MP perturbation theory began only in 1975 with the work of Pople and co-workers and Bartlett and co-workers [R. J. Bartlett, Ann. Rev. Phys. Chem.,31,359 (1981) Hehre et al.]. 

Radionuclides can be roughly divided into a-emitters and P-emitters. The emission of a-particles is associated with a decrease in mass number by four units and in the atomic number by two units. The loss oftwo protons results in an excess oftwo electrons, so the a-decay is accompanied by the emission of electrons (radiation P). In the radioactive P-type conversion, the nucleon in the atom nucleus transforms, a neutron turns into a proton or vice versa, which is associated with the change of particle charge and emitting an electron or positron. The capture of an electron (usually from the sphere K of the electron cloud) by the atom nucleus is also possible, while changing a proton into a neutron. In the electron cloud, the electron from the higher level then jumps to hll the vacant place, which is accompanied by the emission of a photon. ... 

A considerable amount of evidence indicates that nuclear forces are charge-independent, i.e, the neutron-neutron, neutron-proton, and proton-proton forces are identical. The meson theory of nuclear forces, originated by Yukawa, postulates the atomic nucleus being held together by an exchange force in which particles, now called mesons, are exchanged between individual nucleons within the nucleus. 

FIGURE 17.7 When a nucleus ejects an a particle, the atomic number of the atom decreases by 2 and the mass number decreases by 4. The nucleons ejected have been indicated by the blue boundary in the upper part of the diagram. 

The identity of an element depends on the number of protons in the nucleus of the atom. This value is called the atomic number and it is sometimes written as a subscript before the symbol for the corresponding element. Atoms and ions of a given element that differ in number of neutrons have a different mass and are called isotopes. A nucleus with a specified number of protons and neutrons is called a nuclide, and a nuclear particle, either a proton or neutron, may be called a nucleon. The total number of nucleons is called the mass number and may be written as a superscript before the atomic symbol. 

In this chapter we review the recent history of and evidence for collective, moleculelike behavior of valence electrons in atoms and indicate some of the questions that will have to be explored in order to resolve the question of how well the electrons in atoms are described by independent-particle or collective models. We then turn the question around and ask whether atoms in a molecule could, under suitable circumstances, display independent-particle behavior, with their own one-particle angular momenta behaving like nearconstants of the motion. The larger question that emerges is then one of whether few-body systems—the valence electrons of an atom, the atoms that constitute a small polyatomic molecule, and perhaps others such as the nucleons in a nucleus, all of which have heretofore seemed nearly unrelated— share characteristics to the extent that we can devise a unifying picture of the dynamics of few-body systems that will expose their commonalities as well as their obvious differences. 

At the present time, all experimental evidence indicates that nuclei consist of neutrons and protons, which are particles known as nucleons. Nuclei then consist of nucleons. Some of the properties of a neutron, a proton, and an electron, for comparison, are listed in Table 3.2. A free proton—outside the nucleus—will eventually pick up an electron and become a hydrogen atom, or it may be absorbed by a nucleus. A free neutron either will be absorbed by a nucleus or will decay according to the equation... 

I he awmic number or proron number, Z, of an element is the number of protons in the nucleus of an atom of the element. The mass number or nucleon number.. . is the number of protons and neutrons in ibe nucleus of an atom Isoropes of an element differ in mass number but have the same atomic number. Isotopes arc represented as Symbol, e.g. C. Protons are represented as jH. electrons (d-particles) as. neutrons as l,n. and O particles as l(e. In the equation for a nuclear reaction, the sum of the mass numbers is the same on both sides, and the sum of the atomic numbers is the. same on both sides of the equation. 

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