Methane, molecular geometry

Table 7 shows the calculated properties of the molecules in this series. The molecular geometries were inferred from available data for H2S and dimethyl sulphone. Methane... 

We are now ready to account for the bonding in methane. In the promoted, hybridized atom each of the electrons in the four sp3 hybrid orbitals can pair with an electron in a hydrogen ls-orbital. Their overlapping orbitals form four o-bonds that point toward the corners of a tetrahedron (Fig. 3.14). The valence-bond description is now consistent with experimental data on molecular geometry. 

Tetrahedral molecular geometry, with 109.5° bond angles, minimizes repulsion among the bonding electron pairs of methane. ... 

The molecular geometry of methane and of methyl fluoride is tetrahedral. In the case of methane, this symmetrical arrangement of polar covalent carbon-hydrogen bonds leads to a canceling of the bond polarities resulting in a nonpolar molecule. As a nonpolar molecule, the strongest intermolecular force in methane is a London force. In methyl fluoride, a fluorine atom replaces one of the hydrogen... 

Figure 7.7 Molecular geometry of ground-state and (n,Jt ) excited-state methanal molecules...

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. 

A methane molecule is tetrahedral, with bond angles of 109.5°. H 109.5° -P H > P Molecular geometries are / V / described by the relative positions of the nuclei, not the location of electron clouds. NH3 is trigonal pyramidal, not tetrahedral. 

An input file of the molecular geometry indicated in Figure 5 was created as described above. In this geometry two protons of methane are 2.0 A above the plane of ethene one is directly over the center of the carbon-carbon double bond, whereas the other is in a plane normal to the carbon-carbon double bond. The other two protons of methane are more distant from the plane of ethene. After creating the merged file, multiple copies of the file were made. Coordinates of the methane portion of the input file were manipulated in these copies so as to keep ethene (in the XY plane) stationary while the methane molecule was moved over the face of the ethene molecule incrementally in the X and Y directions, keeping the Z distance above the plane of ethene constant. The symmetry of ethene was employed to limit the number of geometries to be calculated. Only one quadrant over one face of... 

B Methane has a molecular geometry that is tetrahedral, while BF3 is trigonal planar in shape and XeF6 is octahedral in shape. 

Let s reconsider the bonding in methane, which has the Lewis structure and molecular geometry shown in Fig. 14.1. In general, we assume that bonding involves only the valence orbitals. This means that the hydrogen atoms in methane use Is orbitals. The valence orbitals of a carbon atom are the 2s and 2p orbitals shown in Fig. 14.2. Thinking about how carbon can use these orbitals to bond to the hydrogen atoms reveals two related problems ... 

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. 

The concept of hybridization of atomic orbitals was subsequently introduced, in an attempt to interpret the difference between the actual bond angle for the water molecule and the value of 90° considered in the previous model. This concept had already been introduced to interpret, for example, the tetrahedral geometry of the methane molecule. We shall come back to this subject later in the chapter, to conclude that, although it is possible to establish a correlation between molecular geometry and hybrid orbitals, it is not correct to take the latter as the basis of an explanation of the former. This distinction is very important in teaching. 

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.

According to the VSEPR model, the molecular geometry about each carbon atom in an alkane is tetrahedral. -= (Section 9.2) The bonding may be described as involving sp -hybridized orbitals on the carbon, as pictured in FIGURE 24.3 for methane. (Section 9.5)... 

A FIGURE 24.3 Bonds about carbon in methane. This tetrahedral molecular geometry is found around all carbons in alkanes. 

The VSEPR model, simple as it is, does a surprisingly good job at predicting molecular shape, despite the fact that it has no obvious relationship to the filling and shapes of atomic orbitals. For example, we would like to understand how to account for the tetrahedral arrangement of C—H bonds in methane in terms of the 2s and 2p orbitals of the central carbon atom, which are not directed toward the apices of a tetrahedron. How can we reconcile the notion that covalent bonds are formed from overlap of atomic orbitals with the molecular geometries that come from the VSEPR model ... 

Figure 9.1 (a) The Lewis structure of the methane molecule, (b) The tetrahedral molecular geometry of the methane molecule. 

Carbon forms four single covalent bonds with four hydrogen atoms. The carbon atom attains an octet, and each hydrogen atom attains a duet—the methane molecule is stable. The molecular geometry of methane can be determined from its Lewis structure using VSEPR theoiy. Since carbon has four electron groups and... 

Note that the structural formula does not convey information about the three-dimensional arrangement of the atoms. To do this, you would need to draw the three dimensional formula depicting the molecular geometry (see Figure 24.3). Methane is a very important molecule since it is the principal component of natural gas. In 2000, more than 2 X 10 of natural gas were consumed in the United States to supply heating, transportation, and industrial needs. 

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