Molecules electron-pair geometry

During the last decade MO-theory became by far the most well developed quantum mechanical method for numerical calculations on molecules. Small molecules, mainly diatomics, or highly symmetric structures were treated most accurately. Now applicability and limitations of the independent particle, or Hartree-Fock (H. F.), approximation in calculations of molecular properties are well understood. An impressive number of molecular calculations including electron correlation is available today. Around the equilibrium geometries of molecules, electron-pair theories were found to be the most economical for actual calculations of correlation effects ). Unfortunately, accurate calculations as mentioned above are beyond the present computational possibilities for larger molecular structures. Therefore approximations have to be introduced in the investigation of problems of chemical interest. Consequently the reliability of calculated results has to be checked carefully for every kind of application. Three types of approximations are of interest in connection with this article. 

Determine the electron-pair geometry of the molecule or ion, using the guidelines in Table 9-2... 

The compound SnCl2. which appears superficially to be similar to Be( l2. actually is different because P - 3, which leads to a triangular coplanar electron-pair geometry. Here, however, BP = 2 and LP = I. and the net result is that SnCI2 is an angular molecule (Figure 9-3) rather than a linear one. 

Still another type of molecule exists for P = 4 water 18 examPle (Flgure 9-7). Here, BP = 2, and LP = 2. The net result is an angular molecule with the two lone pairs occupying tetrahedral positions in the electron-pair geometry. 

A common type of molecule is exemplified by PClr in which = 5. Because all of the pairs are bond pairs, it follows that the molecular geometry will be the same as the electron-pair geometry, a A-bipyramid (Figure 9-9). Note that all of the P-Cl bond distances are the same, but that the Cl-Cl distances (not bonds) are greater between any two Cl atoms in the plane than between an apical Cl... 

The problem is not significantly more difficujt.fpr a molecule such as IFV where P arso e uaIs but BP = 5 and LP = I With an electron-pair geometry... 

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. 

Table 7.2 lists representative structures and examples. One thing to note that will save you some time memorizing information is that for molecules with no nonbonding pairs, the molecular geometry is identical to the electron pair geometry. 

T, T, CE Ammonia has a tetrahedral electron pair geometry. When three of the four electron pairs around the central atom are bonded to three other atoms, the resulting shape of the molecule will be trigonal pyramidal. 

Choices A and B will be tetrahedral in shape while choices C and E will be linear in shape. H2S has an electron pair geometry that is tetrahedral but with only two atoms of H bonded to the tetrahedron, the geometry of the molecule will be bent. 

MOLECULE OF ELECTRON PAIRS BONDING PAIRS LONE PAIRS ELECTRON PAIRS GEOMETRY EXAMPLES... 

Figure 11.11 There are four electron pairs surrounding oxygjen in a vrater molecule. TWo of the electron pairs are used to make covalent bonds to hydrogen within the H2O molecule, while the other two are a ilable to make hydrogen bonds to neighboring molecules. Because the electron-pair geometry is tetrahedral (four electron domains around the central atom), the H — O H bond angle is approximately 109°.

Molecule Bonds Lone Pairs Bond Angle (DEGREES) Molecular Geometry Electron Pair Geometry... 

From Figure 4.1 the electron pair geometry is trigonal planar. Becanse one of the electron pairs is a lone pair, however, which is not connted in the molecular geometry, the SO2 molecule has a bent geometry aronnd the S atom. 

A Each of these molecules has fluorine atoms attached to an atom from Group 1A or 3A to 6A. Draw the Lewis structure for each one and then describe the electron-pair geometry and the molecular geometry. Comment on similarities and differences in the series, (a) BF3 (b) CF4 ... 

The basic idea of VSEPR is that the electron pairs we draw in Lewis diagrams repel each other in real molecules. Therefore they distribute themselves in positions around the central atom that are as far away from each other as possible. These are the locations of lowest potential energy they satisfy the minimization of energy tendency that, we have noted, is one driving force in nature. This arrangement of electron pairs is called electron-pair geometry. The electron pairs may be shared in a covalent bond or they may be lone pairs it makes no difference. 

Predict the electron-pair geometry and shape of a molecule of dichlorine oxide, CI2O. Draw a ball-and-stick representation of the molecule. 

Oxygen has four electron pairs around it, yielding an electron-pair geometry that is tetrahedral. Only two of the electron pairs are bonded to other atoms, so the molecule is bent. The structure is similar to that of water. Even if you drew the correct Lewis diagram as... 

To draw the ball-and-stick representation, start with a sketch of a tetrahedral molecule because the electron-pair geometry is tetrahedral, then erase two atoms and their bonds because only two electron pairs are bonded to atoms. It is preferable to have as many co-planar atoms as possible in your final sketch. Unshared pairs are not shown in a ball-and-stick representation. 

Questions 19 through 30 For each molecule or ion, or for the atom specified in a molecule or ion, write the Lewis diagram, then describe (a) the electron-pair geometry and (b) the molecular geometry predicted by the valence shell electron-pair repulsion theory. Also sketch the three-dimensional ball-and-stick representation of each molecule or ion in Questions 19-22. 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

Figure 7.5 (page 177) shows the geometries predicted by the VSEPR model for molecules of the types AX2 to AX. The geometries for two and three electron pairs are those associated with species in which the central atom has less than an octet of electrons. Molecules of this type include BeF2 (in the gas state) and BF3, which have the Lewis structures shown below ... 

Molecular geometries for molecules with two to six electron-pair bonds around a central atom (A). 

In many molecules and polyatomic ions, one or more of the electron pairs around the central atom are unshared. The VSEPR model is readily extended to predict the geometries of these species. In general—... 

Geometries of molecules such as these can be predicted by the VSEPR model The results are shown in Figure 7.8 (page 181). The structures listed include those of all types of molecules having five or six electron pairs around the central atom, one or more of which may be unshared. Note that—... 

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