Ammonia VSEPR 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. 

Some molecules, such as ammonia, have a lone pair of electrons on the central atom. This electron pair occupies an orbital confined to the central atom. According to the VSEPR model, the four electron pairs in NH3 take up a tetrahedral electron arrangement, so we describe the nitrogen atom in terms of four sp3 hybrid orbitals. Because nitrogen has five valence electrons, one of these hybrid orbitals is already doubly occupied (45). The ls-electrons of the three hydrogen atoms pair with the... 

The VSEPR model is very simple. There are only a few rules to remember, yet the model correctly predicts the molecular structures of most molecules formed from nonmetallic elements. Molecules of any size can be treated by applying the VSEPR model to each appropriate atom (those bonded to at least two other atoms) in the molecule. Thus we can use this model to predict the structures of molecules with hundreds of atoms. It does, however, fail in a few instances. For example, phosphine (PH5), which has a Lewis structure analogous to that of ammonia,... 

The HOMO of NH3 is slightly bonding, because it contains an electron pair in an orbital resulting from interaction of the 2p orbital of nitrogen with the H orbitals of the hydrogens (from the zero-node group orbital). This is the lone pair of the electron-dot and VSEPR models. It is also the pair donated by ammonia when it functions as a Lewis base (discussed in Chapter 6). 

Ammonia, NH3, is used as a fertilizer (injected into the soil) and as a household cleaner (in aqueous solution). Predict the structure of ammonia using the VSEPR model. 

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

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. 

The structure of a primary amine is shown as 27, a secondary amine as 28, and a tertiary amine as 29. Using the VSEPR model (Chapter 3, Section 3.5.4), the three-dimensional shape of each amine with respect to nitrogen is predicted to be similar to that of ammonia. Assume that only the C, H, or N atoms can be observed, but not the electron pair. When viewed, the amine takes on a pyramidal shape with nitrogen at the apex. Indeed, amines are considered to be pyramidal, with the unshared electron pair projected from the apex of the pyramid (see 30). Note that the C-N-C or C-N-H bond angles will vary with the size of the alkyl group. 

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. 

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