Electron-pair geometry

VSEPR electron-pair geometries. The balloons, by staying as far apart as possible, illustrate the geometries (left to right) for two to six electron pairs. 

Click Coached Problems for a self-study species have the general formulas AX2, AX3,..., AX It is understood that there are no unmodule on electron pair geometry. shared pairs around atom A. 

The electron-pair geometry is approximately the same as that observed when only single bonds are involved. The bond angles are ordinarily a little smaller than the ideal values listed in Figure 7.5. 

Next, determine the electron-pair geometry around N using VSEPR rules. Because there are four electrons pairs around N, the electron-pair geometry is tetrahedral (or sp hybridization). 

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

Deteimme the molecular geometry taking into consideration the bond pairs and lone pairs involved in the electron-pair geometry See pp 121-132. 

When P = the tw0 most llke y electron-pair geometries are a A-pynmid... 

When P = 4, the two most likely electron-pair geometries are a n-coplanar structure, in which M lies at the center of the square and in the same plane as the pairs of electrons, which lie at each corner of the square and a tetrahedral structure with M at the center of the tetrahedron outlined by the four pairs of electrons, one pair at each apex of the tetrahedron. Again, for a given length of string, the pairs of electrons will be farther from each other in the tetrahedron, in which the p-M-p angle is 109 28. than in the -coplanar structure, where... 

Now, let us look at molecular geometries or shapes. These are determined by the number (BP) of electron pairs that are used as bond pairs, for these bond pairs will lie in molecular orbitals between M and L (the ligands) in positions determined by / . as we have just discussed. The total number of pairs often (usually) will be equal to the number of ligands attached to M. When this is true (that is, when P = BP), the molecular geometry will be identical to the electron-pair geometry. In the sketches, the electron-pair geometry is shown by shaded planes, and bond pairs of electrons by solid lines. The lone pairs (I P )of... 

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... 

WhenP = 6. as in SF, . the structure is simple, because all the pairs are bond >airs, and the molecular geometry is the same as the electron-pair geometry it s octahedral (Figure lM2). 

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... 

The other electron-pair geometries that are listed in Table 9-2 are also related to specific hybrid molecular orbitals, but they are more complicated because they involve midp. In every case, the ami/ orbitals are of the same priflfap l( BMlSqis%mfeehySflffMftrofif feg... 

Only major resonance forms are considered in providing these approximate answers. The electron-pair geometries on which the sketches are based are referred to by numbers in parentheses. 

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. 

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

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. 

SFj has a trigonal pyramidal molecular geometry with tetrahedral electron pair geometry, BF4 tetrahedral (both molecular and electron pair geometry). 

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°.

FIGURE 5.6 Balloon models of electron-pair geometries for two to six electron pairs. Balloons of similar size and shape, when tied together, naturally assume the arrangements shown. 

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