Molecular structure The three-dimensional arrangement of atoms in a molecule

Molecular structure the three-dimensional arrangement of atoms in a molecule. (13.13)... 

Three-dimensional structures showing shape, geometry, and valance electrons provide a model for comprehending the arrangement of atoms in a molecule. Molecular geometries play an important part in the intramolecular and intermolecular properties of a substance. 

Chirality is a fundamental symmetry property of three-dimensional objects. An object is said to be chind if it cannot be superimposed upon its mirror image. In a chemical context, chirality is applied to the three-dimensional structure of molecules. Many compounds may be obtained in two different forms in which the molecular structures are constitutionally identical but differ in the three-dimensional arrangement of atoms such that they are related as mirror images. In such a case the two possible forms are called enantiomers and are said to be enantiomeric with each other. To take a simple example, the amino acid alanine can be obtained in two forms (1) and (2) which are clearly related as mirror images. 

Figure 3.5 Chemists use the structural formula to show a visual layout of the elements and bonds present in a compound. The molecular model shown here provides a three-dimensional view of the arrangement of atoms in a compound as expressed by the compound s chemical formula. For example, the molecular formula of the methane molecule above illustrates the chemical formula CH. ...

Theoretical chemistry has two problems that remain unsolved in terms of fundamental quantum theory the physics of chemical interaction and the theoretical basis of molecular structure. The two problems are related but commonly approached from different points of view. The molecular-structure problem has been analyzed particularly well and eloquent arguments have been advanced to show that the classical idea of molecular shape cannot be accommodated within the Hilbert-space formulation of quantum theory [161, 2, 162, 163]. As a corollary it follows that the idea of a chemical bond, with its intimate link to molecular structure, is likewise unidentified within the quantum context [164]. In essence, the problem concerns the classical features of a rigid three-dimensional arrangement of atomic nuclei in a molecule. There is no obvious way to reconcile such a classical shape with the probability densities expected to emerge from the solution of a molecular Hamiltonian problem. The complete molecular eigenstate is spherically symmetrical [165] and resists reduction to lower symmetry, even in the presence of a radiation field. 

Have you ever wondered why salt dissolves so quickly in water but oil does not ... why bubbles form when you open a soft drink can ... why a glass of water fizzes when an Alka-Seltzer tablet is plopped into it What s going on at the submicroscopic level that makes these things happen To answer these questions, you need to know more about the structure of water, including the spatial arrangement of atoms in water molecules. The purpose of this section is to begin to describe the three-dimensional structure of molecular compounds such as water. 

So far in this chapter we have considered the Lewis structures of moie-cules. These structures represent the arrangement of the valence electrons in a molecule. We use the word structure in another way when we talk about the molecular structure or geometric structure of a molecule. These terms refer to the three-dimensional arrangement of the atoms in a molecule. For example, the water molecule is known to have the molecular structure... 

These are explained in a number of books and reviews (e.g., Aitken and Kilenyi, 1992 Wade, 1987 Federsel, 1993). Every molecule, indeed every object, has a mirror image. Most molecules such as bromochloromethane, however, can be superimposed on their mirror images. Hence the mirror images of such molecules are not isomers. On the other hand, if the mirror images are not superimposable on each other, they represent different entities and are regarded as isomers. These molecular structures that are constitutionally identical but differ only in the three-dimensional arrangement of their atoms, for example, alanine, constitute a special class of isomers known as enantiomers. 

For a working definition of molecular structure, we have adopted the one given by Eliel The structure of a molecule is completely defined by the number and kind of atoms and the linkages between them . We have applied the term molecular topography to the three-dimensional arrangement of the atoms in a particular conformation the term configuration is also used. The term molecular topology refers to the set of atoms and connections within the molecular skeleton, that is, the molecular structure as defined by Eliel. 

Note that the structural formula does not convey information about the three-dimensional arrangement of the atoms. To do this, you would need to draw the three dimensional formula depicting the molecular geometry (see Figure 24.3). Methane is a very important molecule since it is the principal component of natural gas. In 2000, more than 2 X 10 of natural gas were consumed in the United States to supply heating, transportation, and industrial needs. 

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