VSEPR Water” molecule model

We see that it is a consequence of the Pauli principle and bond formation that the electrons in most molecules are found as pairs of opposite spin—both bonding pairs and nonbonding pairs. The Pauli principle therefore provides the quantum mechanical basis for Lewis s rule of two. It also provides an explanation for why the four pairs of electrons of an octet have a tetrahedral arrangement, as was first proposed by Lewis, and why therefore the water molecule has an angular geometry and the ammonia molecule a triangular pyramidal geometry. The Pauli principle therefore provides the physical basis for the VSEPR model. 

Wilson and Geratt [56] discussed a pair-function model constructing geminals from non-orthogonal one-electron orbitals. Their calculations, performed on the water molecule, supported qualitative valence-shell electron-pair (VSEPR) models [57] of directed valence. 

Figure 14. A three-dimensional representation of the VSCC isosurfaces for the oxide anion of the water molecule. The central white sphere represents the oxide anion. The H atoms are not shown but are in the directions of the two line segments radiating from the oxide anion. The lines coimecting the spheres represent the OH bonds. The spherical envelope centered at the position of the oxide anion represents the 0 e/A isosurface. The two crescent shaped 44 e/A isosurfaces represent local concentrations of electron density centered on the lone pair electrons of the molecule as predicted by die VSEPR model.

We will also begin to correlate the macroscopic properties of molecular compounds with the microscopic properties of their smallest identifiable units, molecules. To this end, we study another model-called vaknce shdl dectron pair repulsion (VSEPR) theory-that predicts the shapes of molecules. For example, VSEPR theory predicts that the two hydrogen atoms and one oxygen atom in the water molecule should have a shape resembling a boomerang. When we examine water in nature, we indeed find that water molecules are shaped like boomerangs. 

The VSEPR model of bonding treats all atoms the same. However, the identities of the atoms in a molecule affect how the electrons are distributed. This knowledge is important, because electron distribution affects the properties of the substance. Life itself depends on the locations of electrons for example, their distribution controls the shape of the DNA double helix and the way it unwinds in the course of reproduction. Electron distributions also control the shapes of our individual proteins and enzymes, and shape is crucial to their function. In fact, when proteins lose their shape—for instance, when we suffer burns—they cease to function and we may die. Knowledge about electron distributions is also essential for understanding less dramatic properties, such as the ability of water to dissolve ionic compounds. 

In the third step, the S03 is absorbed in concentrated sulfuric acid rather than in water because the dissolution of S03 in water is slow. Water is then added to achieve the desired concentration. Commercial concentrated sulfuric acid is 98% H2S04 by mass (18 M H2S04). Anhydrous (100%) H2S04 is a viscous, colorless liquid that freezes at 10.4°C and boils above 300°C. The H2S04 molecule is tetrahedral, as predicted by the VSEPR model (Section 7.9)... 

We can use the example of the balloons to model the shapes that methane (CH4), ammonia (NH3), and water (HgO) assume. As you look at each of these molecules in Figures 1.6-1.8, take note of (1) the number of regions of electron density shown by the Lewis structure, (2) the geometry that is required to maximize the separation of these regions of electron density, and (3) the names of the shapes that result from this treatment using VSEPR. 

The three molecules of interest are methane (4A), ammonia (6A), and water (7A), shown first in the Lewis electron dot representations. Using the VSEPR model, these three molecules are drawn again using the wedge-dashed line notation. Methane (CH4, 4B) has no unshared electrons on carbon but there are electrons in the C-H covalent bonds. Assume that repulsion of the electrons in the bonds leads to a tetrahedral arrangement to minimize electronic repulsion. Ammonia (H3N, 6B) has a tetrahedral array around nitrogen if the electron pair is taken into account. If only the atoms are viewed, however, 6B has the pyramidal shape shown. Water (HOH, 7B) has two electron pairs that occupy the corners of a tetrahedral shape, as shown. 

Water has the structure H-O-H and the VSEPR model assumes that the geometry around oxygen is roughly tetrahedral, so placing two hydrogens and two electron pairs on the oxygen leads to structure 35 for water. The VSEPR model assumes that the electron pairs are invisible, so the atoms will determine the shape. This leads to the conclusion that the H-O-H molecule is bent or angular. 

We can refine the VSEPR model to predict and explain slight distortions of molecules from the ideal geometries summarized in Table 9.2. For example, consider methane (CH4), ammonia (NH3), and water (H2O). All three have tetrahedral electron-domain geometries, but their bond angles differ slightly ... 

In Section 4.3, we learned that the shape of a molecule is an important factor in determining the properties of the substances that it composes. For example, we learned that water would boil away at room temperature if it had a straight shape instead of a bent one. We now develop a simple model called valence shell electron pair repulsion (VSEPR) theory that allows us to predict the shapes of molecules from their Lewis structures. 

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