VSEPR Theory Predicting Molecular Geometries

Use the principles of VSEPR theory and molecular geometry to predict relative melting points, boiling points, and solubilities of compounds. 

When VSEPR theory is used to predict molecular geometries, double and triple bonds are treated identically to single bonds as a single electron group, i.e. as a single place where you can find electrons. 

VSEPR theory The VSEPR (valence shell electron-pair repulsion) theory says that the electron pairs around a central atom will try to get as far as possible from each other in order to minimize the repulsive forces. This theory is used to predict molecular geometry. 

AIM theory provides a physical basis for the theory of Lewis electron pairs and the VSEPR model of molecular geometry. Equipped with computers and computer-generated, three-dimensional electron density maps, scientists are able to view molecules and predict molecular phenomena without even having to get off their chairs ... 

A) The carbon of carbon dioxide has two double bonds. Because there are no unshared pairs of electrons on the central carbon atom, VSEPR theory predicts a linear molecular geometry (type AX2). 

Methane, CH, has four hydrogen atoms bonded to a central carbon atom. Ammonia, NH3, has three hydrogen atoms bonded to a central nitrogen atom. Using VSEPR theory, predict the molecular geometry of each compound. 

The VSEPR theory predicts the three-dimensional shapes of molecules. It is based on simple electrostatics—electron pairs in a molecule will arrange themselves in such a way as to minimize their mutual repulsion. The steric number determines the geometry of the electron pairs (linear, trigonal pyramidal, tetrahedral, and so forth), whereas the molecular geometry is determined by the arrangement of the nuclei and may be less symmetric than the geometry of the electron pairs. 

VSEPR theory predicts that four valence shell electron pairs are directed toward the corners of a regular tetrahedron. That shape gives the maximum separation for four electron pairs around one atom. Thus, VSEPR theory predicts tetrahedral electronic geometry for an AB molecule that has no unshared electrons on A. There are no lone pairs of electrons on the central atom, so another atom is at each corner of the tetrahedron. VSEPR theory predicts a tetrahedral molecular geometry for each of these molecules. 

With the same kind of reasoning, VSEPR theory predicts that sulfite ion, 803 , has tetrahedral electronic geometry. One of these tetrahedral locations is occupied by the sulfur lone pair, and oxygen atoms are at the other three locations. The molecular geometry of this ion is trigonal pyramidal, the same as for other AB3U species. 

By similar reasoning, VSEPR theory predicts octahedral electronic geometry and octahedral molecular geometry for the PFg ion, which has six valence shell electron pairs and six F atoms surrounding one P atom. 

How are double and triple bonds treated when the VSEPR theory is used to predict molecular geometry How is a single unshared electron treated ... 

Solution. The structure of H2O is shown below, where the labels on the H atoms are imaginary. VSEPR theory predicts tetrahedral electron geometry with a bent molecular geometry and bond angles less than 109.5°. The complete set of symmetry operations is , C2, f7y, and (7 2- The multiplication table is shown below. In this symmetry group, each operation is its own inverse. 

Predicting Geometry Using VSEPR Theory Predict the electron and molecular geometry of PCI3. Predict the electron and molecular geometry of the [N03] ion. ... 

The VSEPR model is usually a satisfactory method for predicting molecular geometries. To understand bonding and electronic structure, however, you must look to quantum mechanics. We will consider two theories stemming from quantum mechanics valence bond theory and molecular orbital theory. Both use the methods of quantum mechanics but make different simplifying assumptions. In this section, we will look in a qualitative way at the basic ideas involved in valence bond theory, an approximate theory to explain the electron pair or covalent bond by quantum mechanics. 

In Appendix 2 is outlined the most popular and successful simple model for predicting molecular geometry of main group compounds, the valence shell electron pair repulsion (VSEPR) model. However, alongside it are presented the results of some detailed calculations which prompt the comment the VSEPR model usually makes correct predictions, but there is no simple reason why . The problem of the bonding in transition metal complexes will be the subject of models presented in Chapters 6, 7 and 10 this last chapter reviews the current situation. At this point it is sufficient to comment that the most useful applications of current simple theory are those that start with the observed structure and work from there. In the opinion of the author, the general answer to the question posed at the head of this section is that we really do not know. 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

The result here is quite satisfactory because XeF4 does in fact exhibit square planar geometry. It is worth noting, however, that a square planar shape for XeF4 is also predicted by VSEPR theory. Despite the fact that the molecular orbital method has made some inroads as of late, VSEPR is still the best approach available for rationalizing the molecular geometries of noble gas compounds. 

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