Wednesday, January 22, 2020
Molecular Structures :: essays research papers
Covalent Bonding and Molecular Geometry Objective The objective of this exercise is to help in understanding the geometric relationships of atoms in simple molecules and the relationship of hybridization to the geometry present. Discussion In the last 30 years, data obtained from spectrometric measurements, Xray and electron diffraction studies, and other experiments have yielded precise information about bond distances, angles, and energies. In many cases, the data confirmed conclusions reached earlier. In other cases, valuable new insights were acquired. Structure theory has advanced far beyond the simple electron dot representations and now rests securely on the foundations of quantum and wave mechanics. Although problems involving only simple molecules can now be solved with mathematical rigor, approximations such as the valence bond theory and the molecular orbital theory are very successful in giving results that agree with experimental measurements. This exercise will use valence bond theory or hybridization to look at the geometry formed from various hybridizations. You will use a framework model kit which gives the correct angles for the each of these hybridizations. The first bond formed between any two atoms is always a sigma (s)bond (one that is symmetric about the bond axis). Additional bonds between the same two atoms will be pi (p)bonds (perpendicular to the bond axis). It is the sigmabonds and any lonepairs of electrons occupying the sigma hybrid orbitals that determine the geometry of a molecule. Pibonds are always perpendicular to the sigmabonds and follow the geometry formed by the sigmabonding. Procedure Check out a molecular model kit from the stockroom. Read the kit directions to see which framework center is used for each hybridization. Tetrahedral (sp3 hybridization) CH4 Construct a model of methane using a tetrahedral center (4 prongs) and four rods of the same color to show how the 4 H's are attached. Geometry     Lewis dot diagram     # of s bonds on C     Approximate H-C-H angel     Max # atoms (incl. C) in one plane     Is there a mirror plane(divides the molecule in equal halves) ? H3CCH3 Construct a model of ethane using a tetrahedral center for each C and the same color rods for all 6 H's with a CC bond present. Geometry     Lewis dot diagram     # of s bonds on each C     Approximate H-C-H angle     Approximate H-C-H angle The C-C bond is a single bond and has free rotation about it. Arrange the ethane molecule so that each CH bond on one C atom is exactly parallel to a CH bond      on the second C atom. (This is the eclipsed position.) View this arrangement by looking along the CC bond such that the atoms on the front C blank out those on the back C.
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