Atomic Structure The Nucleus

Not all carbon compounds are derived from living organisms, of course, and chemists over the years have developed a remarkably sophisticated ability to design and synthesize new organic compounds. Medicines, dyes, polymers, food additives, pesticides, and a host of other substances are now prepared in the laboratory. Organic chemistry touches the lives of everyone. Its study is a fascinating undertaking. 

We ll ease into the study of organic chemistry by first review-ing some ideas about atoms, bonds, and molecular geometry that you may recall from your general chemistry course. Much of the material in this chapter and the next is likely to be familiar to you, but it s nevertheless a good idea to make sure you understand it before going on. 

Although extremely small—about to meter (m) in diameter— 

Volume around nucleus occupied by orbiting electrons 

Although extremely small—about 10 to 10 l meter (m) in diameter— the nucleus nevertheless contains essentially all the mass of the atom. Electrons have negligible mass and circulate around the nucleus at a distance of approximately 10 m. Thus, the diameter of a typical atom is about 2 X 10 m, or 

Mgure 1.2 A schematic view of an atom. The dense, positively charged nucleus contains most of the atom s mass ana is surrounded by negatively charged electrons. The three-dimensional view on the right shows calculated electron-density surfaces. Electron density increases steadily toward the nucleus and is 40 times greater at the blue solid surface than at the gray mesh surface. 

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The survey of the general scene begins with certain material assemblies which are remote at any rate from terrestrial concern. According to our conception of atomic structure the nucleus is small and dense. Stripped of their electrons at immensely high temperatures, nuclei constitute systems in which enormous concentrations... 

Atomic Structure The Nucleus Atomic Structure Orbitals 4 Atomic Structure Electron Configurations 6 Development of Chemical Bonding Theory 7 The Nature of Chemical Bonds Valence Bond Theory sp Hybrid Orbitals and the Structure of Methane 12 sp Hybrid Orbitals and the Structure of Ethane 13 sp2 Hybrid Orbitals and the Structure of Ethylene 14 sp Hybrid Orbitals and the Structure of Acetylene 17 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur 18 The Nature of Chemical Bonds Molecular Orbital Theory 20 Drawing Chemical Structures 21 Summary 24... 

A representation of atomic structure. The various spheres are not drawn to scale. The lump of iron on the left would contain almost a million million million million (10 ) atoms, one of which is represented by the sphere in the top center of the page. In turn, each atom is composed of a number of electrons, protons, and neutrons. For example, an atom of the element iron contains 26 electrons, 26 protons, and 30 neutrons. The physical size of the atom is determined mainly by the number of electrons, but almost all of its mass is determined by the number of protons and neutrons in its dense core or nucleus (lower part of figure). The electrons are spread out around the nucleus, and their number determines atomic size but the protons and neutrons compose a very dense, small core, and their number determines atomic mass. 

Shortly after coming to Rutherford s laboratory, Bohr set to work on the problem of understanding the structure of atoms. Rutherford s discovery of the atomic nucleus had introduced formidable problems. It seemed necessary to assume that the electrons in an atom orbited the nucleus. Otherwise, the electrical attraction between the electrons and the nucleus would cause the electrons and the nucleus to collide with one another. But, as we have seen, the assumption that the electrons orbited the nucleus didn t seem to work either. Orbiting electrons should lose energy and fall into the nucleus anyway. 

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

Growth of Nuclei to Metal Nanoparticles. If the elemental cluster of 13 atoms is the nucleus, the growth of nuclei to metal nanoparticles could proceed by deposition of atoms or microclusters on the surface of nuclei. This process is understandable based on the consideration of the formation of monodispersed nanoparticles. However, structural analysis has often proposed the aggregation of elemental clusters to form fundamental clusters (64). A similar idea is discussed for the structural analysis of bimetallic nanoparticles with cluster-in-cluster structure (40,61). 

In these Lewis structures, the symbol of the atom represents the nucleus and all filled valence shells. Only electrons in the valence shell are shown. 

According to Rutherford s atomic structure, the protons inside an atom are all gathered at the centre (nucleus) of the atom, with the electrons scattered randomly around, as the fruit of a peach is formed around the stone in the center, or the solar system, in which the sun is the nucleus and planets are electrons. 

However, all standard NMR parameters are of short-ranged nature chemical shifts are typically affected by the first and second sphere of atoms surrounding the nucleus and usually provide only qualitative measures 3/-couplings are limited to dihedral angles via three covalent bonds and through space connectivities via NOE can only be found up to 5 A in favourable cases. As soon as the chain of short-range information is interrupted by NMR-inactive nuclei or a flexible linker, distant parts of a molecule cannot be correlated and the structure determination is bound to fail. 

In the nitroso derivatives of the tertiary amines, the characteristic group is linked to a carbon atom in the nucleus. The compound formed by the action of nitrous acid on dimethyl-aniline, for example, has the structure indicated by the formula, N0.C6H4.N(CH3)2. 

Since the time of Thomson and Rutherford, a great deal has been learned about atomic structure. The simplest view of the atom is that it consists of a tiny nucleus (about 10 cm in diameter) and electrons that move about the nucleus at an average distance of about 10 cm from it (see Figure 3.5). To visualize how small the nucleus is compared with the size of the atom, consider that if the nucleus were the size of a grape, the electrons would be about one mile away on average. 

In a similar maimer, patterns of nuclear stability, results of nuclear reactions and spectroscopy of radiation emitted by nuclei have yielded information which helps us develop a picture of nuclear structure. But the situation is more complicated for the nucleus than for the atom. In the nucleus there are two kinds of particles, protons and neutrons, packed close together, and there are two kinds of forces - the electrostatic force and the short range strong nuclear force. This more complex situation has caused slow progress in developing a satisfactory model, and no single nuclear model has been able to explain all the nuclear ph omena. 

In this model the atom is called a nuclear atom because the positive charge is localized in a small, compact structure (the nucleus) and not spread out uniformly, as in the plum pudding view. 

The Structure of the Atom Through experimentation in the nineteenth and early twentieth centuries, scientists have learned that an atom is composed of three elementary particles proton, electron, and neutron. The proton has a positive charge, the electron has a negative charge, and the neutron has no charge. Protons and neutrons are located in a small r ion at the center of the atom, called the nucleus, and electrons are spread out about the nucleus at some distance from it. 

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