Electron-pair angle

Atoms Electron Pairs Angle Structure Example... 

Earlier, we drew Lewis diagrams in which carbon, nitrogen, or oxygen was the central atom. In all cases the central atom was surrounded by four pairs of electrons. In Section 12.7 we showed how beryllium and boron—also Period 2 elements—do not conform to the octet rule. The beryllium atom in BeF2 is flanked by only two electron pairs in BEj, the boron atom has three electron pairs around it. Our question then is this How do two, three, or four electron pairs distribute themselves around a central atom so they are as far apart as possible This question is answered by identifying the electron-pair angle, the angle formed by any two electron pairs and the central atom. 

When electron pairs are as far apart as possible, all electron-pair angles around the central atoms are equal. The electron-pair geometries that result from two, three, or four electrons pairs are shown in Figure 13.1. The bond angles are derived by geometry. 

Electron Pairs Geometry Electron-Pair Angles... 

Electron-pair angle Electron-pair geometry Linear... 

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... 

The H—O—H angle m water (105°) and the H—N—H angles m ammonia (107°) are slightly smaller than the tetrahedral angle These bond angle contractions are easily accommodated by VSEPR by reasoning that electron pairs m bonds take up less space than an unshared pair The electron pair m a covalent bond feels the attractive force of... 

Physical Properties. Sulfur tetrafluoride has the stmcture of a distorted trigonal bipyramid, the sulfur having hybrid sp d orbitals and an unshared electron pair (93). The FSF bond angles have been found to be 101° and 187°, and the bond distances 0.1646 and 0.1545 nm (94). 

The triply connected phosphoms compounds have a lone electron pair that dominates much of the chemistry for these compounds. Triply connected compounds typically exhibit pyramidal symmetry arising fromp hybridization. A considerable amount of sp character may be present as well. Bond angles range near 100° vs 90° theoretical. Tricoordinate compounds typically act as electron donors, forming metal coordination compounds and addition compounds such as H P BF [41593-56-0]. 

Figure 2.3 The shapes of orbitals for the s electron pair, the three pairs of p electrons with obitals mutally at right angles, and the sp orbitals which have the major lobes pointing towards the apices of a regular tetrahedron.

The typical properties of water arise from the ability of the water molecule to participate in four hydrogen bonds due to its two protons and its two lone electron pairs (2s)2 (2pz)2 which act as proton acceptors. In the condensed state, the angle between the 2px and the 2py orbital of oxygen is enlarged by hydridisation to a mixture of s- and p-state to 109°. Because both of the free electron pairs are situated in a plane... 

As we saw in A Preview of Carbonyl Compounds, the most general reaction of aldehydes and ketones is the nucleophilic addition reaction. A nucleophile, Nu-, approaches along the C=0 bond from an angle of about 75° to the plane of the carbonyl group and adds to the electrophilic C=0 carbon atom. At the same time, rehybridization of the carbonyl carbon from sp2 to sp3 occurs, an electron pair from the C=0 bond moves toward the electronegative oxygen atom, and a tetrahedral alkoxide ion intermediate is produced (Figure 19.1). 

Two electron pairs are as far apart as possible when they are directed at 180° to one another. This gives BeF2 a linear structure. The three electron pairs around the boron atom in BF3 are directed toward the comers of an equilateral triangle the bond angles are 120°. We describe this geometry as trigonal planar. 

Species type Orientation of electron pairs Predicted bond angles Example Ball and stick model... 

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. 

Each carbon atom behaves as if it were surrounded by two electron pairs. Both of the bond angles (H—C=C and C=C—H) are 180°. The molecule is linear the four atoms are in a straight line. The two extra electron pairs in the triple bond do not affect the geometry of the molecule. 

To illustrate this rule, consider the ethylene (C2H4) and acetylene (C2H2) molecules. You will recall that the bond angles in these molecules are 120° for ethylene and 180° for acetylene. This implies sp2 hybridization in C2H4 and sp hybridization in C2H2 (see Table 7.4). Using blue lines to represent hybridized electron pairs,... 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

A sulfur hexafluoride molecule, SF6, has six atoms attached to the central S atom and no lone pairs on that atom (8). According to the VSEPR model, the electron arrangement is octahedral, with four pairs at the corners of a square on the equator and the remaining two pairs above and below the plane of the square (see Fig. 3.2). An F atom is attached to each electron pair, and so the molecule is predicted to be octahedral. All its bond angles are either 90° or 180°, and all the F atoms are equivalent. 

Data for which no reference is given are from the Slrukturbericht of P. P. Ewald and C. Hermann. 6 R. W. G. Wyckoff, Z. Krisl., 75,529 (1930). W. H. Zachariasen, ibid., 71, 501, 517 (1929). d The very small paramagnetic susceptibility of pyrite requires the presence of electron-pair bonds, eliminating an ionic structure Fe++S2. Angles are calculated for FeS2, for which the parameters have been most accurately determined. The parameter value (correct value = 0.371) and interatomic distances for molybdenite are incorrectly given in the Slrukturbericht. 

When there is electronic resonance, the bond lengths, bond angles, and nuclear positions are intermediate between those corresponding to the individual resonance structures, as found in the crystal structure of the tin dimer.32 This type of bond, that is, a single bond plus a resonating unshared electron pair, was subsequently found to occur on the 100 surfaces of silicon.33... 

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