Shape central atom concepts

The concept of the c entralatom is convenient to use in discussing the shapes of molecules. In a simple molecule, one of the atoms usually is "central" to the whole molecule. For example, in CC14 the central atom is C, the one to which all the other atoms are attached. 

Some of the chemical concepts with little or no quantum-mechanical meaning outside the Bohmian formulation but, well explained in terms of the new interpretation, include electronegativity, the valence state, chemical potential, metallization, chemical bonding, isomerism, chemical equilibrium, orbital angular momentum, bond strength, molecular shape, phase transformation, chirality and barriers to rotation. In addition, atomic stability is explained in terms of a simple physical model. The central new concepts in Bohmian mechanics are quantum potential and quantum torque. 

To predict the shapes of a variety of molecules we will use a theory which postulates that the shapes of molecules depend on the total number of bonded, and nonbonded or lone electron pairs surrounding a central atom. The chief concept is that electrons tend to repel each other, and that the mutual repulsion of all the electron pairs results in the molecules shape. The name of the method is therefore the valence-shell electron-pair repulsion theory, or the VSEPR theory. 

With XeF4, a new factor appears the two pairs of electrons can be arranged in two different ways about die central atom. The axes of the orbitals with the unshared pairs of electrons can form either an angle of 90 (Fig. 19.20d) or an angle of 180 (Fig. 19.20c). The shape of XeF4 is square-planar (Fig. 19.20c). The Xe atom is in the center and the F atoms are at the comers of a square all the atoms are thus in the same plane. Evidendy, the two orbitals with the unshared pairs form the maximum angle of 180 with each other, as we would expect from our concept of electron pair repulsion. 

The concept of conformational isomerism is central to any consideration of molecular shape. Molecules that are flexible may exist in many different shapes or conformers. Conformational isomerism is the process whereby a single molecule undergoes transitions from one shape to another the physical properties of the molecule have not changed, merely the shape. Conformational isomerism is demonstrated by compounds in which the free rotation of atoms around chemical bonds is not significantly hindered. The energy barrier to the transition between different conformations is usually very low... 

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