Methane VSEPR

The tetrahedral geometry of methane is often explained with the valence shell electron pair repulsion (VSEPR) model The VSEPR model rests on the idea that an electron pair either a bonded pair or an unshared pair associated with a particular atom will be as far away from the atom s other electron pairs as possible Thus a tetrahedral geomehy permits the four bonds of methane to be maximally separated and is charac terized by H—C—H angles of 109 5° a value referred to as the tetrahedral angle... 

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

We now have three substances remaining methane, CH4, methyl fluoride, CH3F, and krypton difluoride, KrF2. We also have two types of intermolecular force remaining dipole-dipole forces and London forces. In order to match these substances and forces we must know which of the substances are polar and which are nonpolar. Polar substances utilize dipole-dipole forces, while nonpolar substances utilize London forces. To determine the polarity of each substance, we must draw a Lewis structure for the substance (Chapter 9) and use valence-shell electron pair repulsion (VSEPR) (Chapter 10). The Lewis structures for these substances are ... 

For a final example, let s consider methane, CH with four bonding electrons surrounding the central carbon atom. VSEPR predicts a tetrahedral arrangement with bond angles of 109.5°. 

Real molecules have all sorts of symmetrical shapes that just don t make sense if electrons truly occupy only pure orbitals (like s and p). The mixing of pure orbitals into hybrids allows chemists to explain the symmetrical shapes of real molecules with VSEPR theory. This kind of mixing must in some sense actually occur, as the case of methane, CH, makes clear. 

The single 2s orbital combines with the three 2p orbitals to create four identical sp hybrid orbitals. The fact that each sp orbital is identical is important because VSEPR theory can now explain the symmetrical shape of methane the tetrahedron. 

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. 

Although this phenomenon represents an exception to the rules, it s somewhat less annoying than other exceptions because hybridization allows for the nicely symmetrical orbital geometries of actual atoms within actual molecules. VSEPR theory presently clears its throat to point out that the negative charge of the electrons within the hybridized orbitals causes those equivalent orbitals to spread as far apart as possible from one another. As a result, the geometry of sp -hybridized methane (CH ), for example, is beautifully tetrahedral. 

Q Draw the box diagrams associated with the thought experiment which describes the VSEPR approach to the determination of the methane structure. 

The same kind of sp3 hybridization that describes the bonds to carbon in the tetrahedral methane molecule also describes bonds to nitrogen in the trigonal pyramidal ammonia molecule, to oxygen in the bent water molecule, and to all other atoms that VSEPR theory predicts to have a tetrahedral arrangement of four charge clouds. 

Organic molecules have specific three-dimensional shapes, which can be predicted by the VSEPR model (Section 7.9). When carbon is bonded to four atoms, as in methane, the bonds are oriented toward the four corners of a tetrahedron with carbon in the center and with H-C-H angles near 109.5° ... 

To see how the VSEPR model works, examine the methane (CH4) molecule. The first step is to write its Lewis structure. 

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. 

Methane, CH4, has steric number 4, and VSEPR predicts a tetrahedral structure, which is confirmed by experimental results. Starting with the electron configuration C (ls) (2s) (2p), the VB model cannot account for the formation of CH4 and predicts that CH2 would be the stable hydride, which is again contrary to the... 

Method must be applied separately to these two centres. Thus in the case of ethane both centres are C atoms which have identical tetrahedral stereochemistries, determined in precisely the same way as for the methane molecule in Figure 6.4, example 1, and for this reason the calculation need not be described separately. In the same way, the stereochemistry of any C atom centre in any complex organic molecule may be predicted. However, it must still be remembered that VSEPR theory cannot be used to determine the stereochemistry of transition metal complexes, owing to the presence of an incomplete d-subshell. 

VSEPR theory has been used to determine the shape of the methane molecule. Figure 6.4, example 1, as a tetrahedral stereochemistry about the carbon atom. While this simple theory predicts the correct shape, it does this on the basis of a simple numerical count, without considering... 

One of the most familiar compounds of carbon is methane, CH, the main component of natural gas. The methane molecule consists of a carbon atom with four hydrogen atoms bound to it in a tetrahedral fashion. That is, as predicted by the VSEPR model (see Chapter 12), the four pairs of bonding electrons around the carbon have minimum repulsions when they are located at the corners of a tetrahedron. 

Problem 3.18. What does VSEPR theory predict for the structure of methane, CH4 ... 

Thinking it Through The first step in applying the VSEPR method is to count the number of electron regions around the central atom. In this case, the Lewis structure is given, showing four pairs of electrons around the carbon atom. These electron pairs are all used to form bonds, and there are no lone pairs. Electron pair repulsion is minimized when the four electron pairs form a tetrahedron around the carbon atom, choice (D). Observe the similarity to methane, CH4, which was probably the first tetrahedral molecule you studied. 

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 ... 

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)... 

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