Nuclei, atomic

The atomic nucleus is a tiny massive entity at the center of an atom. Occupying a volume whose radius is 1/100,000 the size of the atom, the nucleus contains most (99.9%) of the mass of the atom. In describing the nucleus, we shall describe its composition, size, density, and the forces that hold it together. After describing the structure of the nucleus, we shall go on to describe some of the limits of nuclear stability. 

The nucleus is composed of protons (charge = +1 mass = 1.007 atomic mass units ([p.]) and neutrons. The number of protons in the nucleus is called the atomic number Z and defines which chemical element the nucleus represents. The number of neutrons in the nucleus is called the neutron number N, whereas the total number of neutrons and protons in the nucleus is referred to as the mass number A, where A = N -I- Z. The neutrons and protons are referred to collectively as nucleons. A nucleus with a given N and Z is referred to as a nuclide. Nuclides with the same atomic number are isotopes, such as and whereas nuclides with the same N, such as and are called isotones. Nuclei such as N and which have the same mass number, are isobars. Nuclides are designated by a shorthand notation in which one writes Chemical Symbol, that is, for a nucleus with 6 protons and 8 neutrons, one writes gCs, or, g C, or just The size of a nucleus is approximately 1 to 10 x 10 m, with the nuclear radius being represented more precisely as 1.2 x A x 10 m. We can roughly approximate the nucleus as a sphere and thus we can calculate its density 

Current research on nuclei, their properties, and the forces that hold them together focuses on studying nuclei at the limits of stability. The basic idea is that when one studies nuclei under extreme conditions, one then has a unique ability to test theories and models that were designed to describe the normal properties of nuclei. One limit of nuclear stability is that of high Z, that is, as the atomic number of the nucleus increases, the repulsion between the nuclear protons becomes so large as to cause the nucleus to spontaneously fission. The competition between this repulsive Coulomb force and the cohesive nuclear force is what defines the size of the Periodic Table and the number of chemical elements. At present there are 112 known chemical elements, and evidence for the successful synthesis of elements having the atomic numbers 114 and 116 has been presented. 

Kaltsoyannis, Nikolas, and Scott, Peter (1999). The f Elements. New York Oxford University Press. 

Both fusion and fission reactions can be used in bombs. The fusion re- A nuclear explosion at sea. actions require a very high temperature to get started, so they are initiated by fission reactions. (When controlled at slower rates in nuclear reactors, fission reactions are used to produce power and additional nuclear fuel.) 

Chemical explosions may be either decomposition or combination reactions. 

In either case, the reaction is exothermic and the energy released by the reaction is partially converted to work. Decomposition reactions occur in materials such as trinitrotoluene (TNT) and nitroglycerine. The molecules of these materials contain oxygen. When the molecule decomposes, the products are 

Combination reactions require that two or more components react together exothermically to produce hot gases. Some examples are ammonium nitrate and fuel oil (ANFO), gunpowder (potassium nitrate, carbon, and sulfur), and fireworks. In these explosions, the reactants that make up the explosive must be carefully mixed to assure that the reaction will continue. 

Clearly, the properties of the electron posed problems about the inner structure of atoms. If everyday matter is electrically neutral, the atoms that make it up must be neutral also. But if atoms contain negatively charged electrons, what positive charges balance them And if an electron has such an incredibly tiny mass, what accounts for an atom s much larger mass To address these issues, Thomson proposed a model of a spherical atom composed of diffuse, positively charged matter, in which electrons were embedded like raisins in a plum pudding.  

Near the turn of the 20 century, French scientists discovered radioactivity, the emission of particles and/or radiation from atoms of certain elements. Just a few years later, in 1910, the New Zealand-born physicist Ernest Rutherford (1871-1937) used one type of radioactive particle in a series of experiments that solved this dilemma of atomic structure. 

A Hypothesis Expected result based on plum pudding model 

Cross section of gold foil composed of plum pudding atoms 

EXPERIMENT a particles emit a flash of light when they pass through the gold atoms and hit a phosphor-coated screen. 

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Mosshauer effect The resonance fluorescence by y-radiation of an atomic nucleus, returning from an excited state to the ground state. The resonance energy is characteristic of the chemical environment of the nucleus and Mossbauer spectroscopy may be used to yield information about this chemical environment. Used particularly in the study of Fe. Sn and Sb compounds. 

Evans R D 1955 The Atomic Nucleus (New York McGraw-Hill)... 

Figure Bl.9.1. Diagrams showing that x-ray and light scattering involve extra-nuclear electrons, while neutron scattering depends on the nature of the atomic nucleus.

Ch em uses Slater atom ic orbitals to con struct sent i-em pirical molecular orbitals. I he complete set of Slater atomic orbitals is called the basis set. Core orbitals are assumed to be chemically inactive and arc not treated explicitly. Core orbitals and the atomic nucleus form the atomic core. 

The electron is the lightweight particle that "orbits" outside of the atomic nucleus. Chemical bonding is essentially the interaction of electrons from one atom with the electrons of another atom. The magnitude of the charge on an electron is equal to the charge on a proton. Electrons surround the atom in pathways called orbitals. The inner orbitals surrounding the atom are spherical but the outer orbitals are much more complicated. 

A neutron is characterized by having no electrical charge but has one unit of atomic mass, the same as that of a proton (Figure 46.2). Neutrons, like protons, reside in the atomic nucleus and contribute to the mass of the atom. The chemistry of an atom, like its size, is determined by the electrons in the atom. The mass of the atom is characterized mainly by the total number of neutrons and protons in the nucleus (atomic binding energies are ignored in this discussion). For mass spectrometric purposes of measurement, it is the mass that is important in establishing m/z values. 

While the number of protons in an atomic nucleus characterizes each element, the mass of the nucleus comprises the total number of protons and neutrons. 

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Enetgy-Loss Spectroscopy, HREELS, uses electron impact. The fourth technique. Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. 

RBS is based on collisions between atomic nuclei and derives its name from Lord Ernest Rutherford who first presented the concept of atoms having nuclei. When a sample is bombarded with a beam of high-energy particles, the vast majority of particles are implanted into the material and do not escape. This is because the diameter of an atomic nucleus is on the order of 10 A while the spacing between nuclei is on the order of 1 A. A small fraction of the incident particles do undergo a direct collision with a nucleus of one of the atoms in the upper few pm of the sample. This collision actually is due to the Coulombic force present between two nuclei in close proximity to each other, but can be modeled as an elastic collision using classical physics. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

At this point the nomenclature used in XPS and AES should be explained. In XPS the spectroscopic notation is used, and in AES the X-ray notation. The two are equivalent, the different usage having arisen for historical reasons, but the differentiation is a convenient one. They are both based on the so-called j-j coupling scheme describing the orbital motion of an electron around an atomic nucleus, in which the... 

ATOMIC NUMBER The number of protons in an atomic nucleus. 

Decay The spontaneous disintegration of an unstable atomic nucleus to form another more stable element or isotope of a lower atomic mass. 

The potential energy component is the Coulomb repulsion between each pair of charged entities (treating each atomic nucleus as a single charged mass) ... 

E. P. Wigner (Princeton) the theory of the atomic nucleus and elementary particles, particularly through the discovery and application of fundamental symmetry principles. 

A. Bohr (Copenhagen), B. Mottelson (Copenhagen) and J. Rainwater (New York) discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection. 

A convenient orbital method for describing eleetron motion in moleeules is the method of molecular orbitals. Molecular orbitals are defined and calculated in the same way as atomic orbitals and they display similar wave-like properties. The main difference between molecular and atomic orbitals is that molecular orbitals are not confined to a single atom. The crests and troughs in an atomic orbital are confined to a region close to the atomic nucleus (typieally within 1-2 A). The electrons in a molecule, on the other hand, do not stick to a single atom, and are free to move all around the molecule. Consequendy, the crests and troughs in a molecular orbital are usually spread over several atoms. 

Atom-kem, m. atomic nucleus, -kette,/. chain of atoms, atomic chain, -lage, /. atomic layer atomic position, -lehre, /, doctrine of atoms, atomic theory, -mechanik, /. mechanics of the atom, -modell, n, atomic model, -nummer, /, atomic number, -ord-nung, /. atomic arrangement, -refraktion, /. atomic refraction, -rest, m. atomic residue (= Atomrumpf). -ring, m. ring of atoms, -rumpf, m. atomic residue or core (remainder of an atom, as after removal of some electrons), -schale, /, atomic shell, -strabl, m. atomic ray, -tafel, /, atomic table, atomtbeoretisch, a. of or according to the atomic theory,... 

Mit-bewegung, /. associated movement, comovement relative motion (as of the atomic nucleus), -bewerber, m. competitor, mit-einander, adv. with one another, together. 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

As far as is known, ordinary matter is made of tiny building blocks called elementary particles. For example, an atom is made up of a nucleus surrounded by one or more electrons. As far as scientists have been able to determine, the electrons are elementary particles, not made of anything simpler. Fdowever, an atomic nucleus is not clcmcntai y, but is a composite particle made up of simpler particles called protons and neutrons. (The lightest nucleus is the nucleus of ordinai y hydrogen, which consists of only a single proton.) Today, physicists believe that even protons and neutrons are not elementai y but are composite particles made up of still simpler building blocks called quarks. 

Nuclear energy, sometimes referred to as atomic energy, originates in the atomic nucleus, which is the extremely dense core at the heart of an atom. A large... 

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