VSEPR theory methane

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. 

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. 

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... 

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

We can rationalize the difference between prediction and experiment with VSEPR theory by noting that the electronegativity of chlorine is greater than that of carbon (Table 1.12). In a methane C-H bond, the carbon atom and the hydrogen atom attract the electron pair approximately equally. In CH3CI, however, the electrons in the C-Cl bond will be pulled toward the... 

The theoretical basis for a molecule possessing a particular geometric shape is the concept that electron pairs, whether they are part of a covalent bond (as in a bonding pair) or not (as in a nonbonding pair), will repel each other. In methane, water, and all other molecules, this repulsion means that the electron pairs will get as far away from each other as they can get. The general name for this theory is the valence-shell electron-pair repulsion (VSEPR) theory. 

Examine the ball-and-stick models for water and methane as drawn in Figure 6.23. In terms of the VSEPR theory, how are these models the same and how are they different ... 

If we do not use hyperconjugation, how are we to describe where the "extra" electrons go This question arises because we seem implicitly to think about bonding in terms of hybridization of atomic orbitals. Thus, in order to describe bonding in the methane molecule consistent with its tetrahedral geometry we introduced the concept of sp hybrid orbitals. That is, the hybridization was introduced because of the geometry. The geometry of a molecule can be determined only experimentally or estimated by using VSEPR theory. 

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. 

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