Ammonia molecular geometry

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. 

Pyramidal AX3E species, such as ammonia (NH3), have a pyramidal geometry (Figure 7.12). The four pairs of electrons (three bonding pairs and one lone pair) approximately point to the vertices of a tetrahedron. As with the bent species, however, only the atoms are considered in describing the molecular geometry. 

As one can see, the SLG-MINDO/3 in general improves the description of molecular geometry as compared to the SCF-MINDO/3 method. Obvious deterioration takes place only for the ammonia molecule (particularly for the valence angle). However, in other cases, transition to the SLG wave function improves the values of the valence angles. This is seen in the example of the hydrogen peroxide molecule. 

As you look at this Lewis structure, notice that there are four pairs of electrons. There are three shared pairs, denoted by the lines, and one unshared pair, represented by the dots above the N atom. The unshared pair is also called a lone pair. Ammonia s four pairs of electrons are all valence electrons. The shape that allows these four pairs of electrons to be as far from each other as possible places them at the corners of a tetrahedron, as shown in Figure 9.6. This is called electron group geometry. The arrangement of the atoms is called molecular geometry, which in this case is pyramidal. 

In Figure 9.6, a diagram of the ammonia molecule, notice that the molecular geometry is pyramidal because that is how its atoms are arranged in space. However, its electron pair geometiy is tetrahedral and it is the electron pair geometry that dictates the molecular geometry. 

Ammonia has a trigonal pyramidal molecular geometry BECAUSE ammonia has a tetrahedral electron pair geometry with three atoms bonded to the central atom. 

Quantum chemical methods are valuable tools for studying atmospheric nucle-ation phenomena. Molecular geometries and binding energies computed using electronic structure methods can be used to determine potential parameters for classical molecular dynamic simulations, which in turn provide information on the dynamics and qualitative energetics of nucleation processes. Quantum chemistry calculations can also be used to obtain accurate and reliable information on the fundamental chemical and physical properties of molecular systems relevant to nucleation. Successful atmospheric applications include investigations on the hydration of sulfuric acid and the role of ammonia, sulfur trioxide and/or ions... 

The rules and principles of molecular geometry accurately predict the shapes of simple molecules such as methane (CH4), water (H2O), or ammonia (NH3). As molecules become increasingly complex, however, it becomes very difficult, but not impossible, to predict and describe complex geometric arrangements of atoms. The number of bonds between atoms, the types of bonds, and the presence of lone electron pairs on the central atom in the molecule critically influence the arrangement of atoms in a molecule. In addition, use of valance shell electron pair repulsion theory (VSEPR) allows chemists to predict the shape of a molecule. 

Fig. 1.38. Contour maps of L for methane, ammonia, and water. For water, the contours are in the plane of the molecule. For ammonia and methane the contours are in the plane that bisects the molecule with a hydrogen above and below the plane. Reproduced with permission from R. J. Gillespie and P. L. A. Popelier, Chemical Bonding and Molecular Geometry, Oxford University Press, Oxford, 2001, p. 172.

Figure 23.9 Geometry of ammonia, (a) Tetrahedral orbital geometry, (b) Pyramidal molecular geometry.

Kristof et al. [246] proposed an empirical force field, fitted to experimental molecular geometry and vapor-liquid equilibrium properties. This force field consists of one U 12-6 site plus four partial charges. Recently, Zhang and Siepmann [247] proposed a five-site ammonia force field based on the geometry of the Impey and Klein [108] model. This force field also consists of one LJ 12-6 site and four partial charges, three of them located at the hydrogen positions and one located at a... 

What is the difference between the electron-domain geometry and the molecular geometry of a molecule Use the ammonia molecule as an example in your discussion. 

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