Bonding pair VSEPR theory

The shapes of covalent compounds are determined by the tendency for bonding pairs to be as far apart as possible whilst lone pairs have a greater effect than bonding pairs (VSEPR theory). 

Steric number n. The sum of the number of bonds and lone pairs around a bonded atom (VSEPR theory). 

Next we will consider molecules that have both bonded and nonbonded pairs of electrons in the valence shell of the central atom. Water and ammonia have four electron pairs around the central atom. Some of the electron pairs in water and ammonia are bonded to hydrogen atoms, but the central atom also has unshared electron pairs. VSEPR theory describes the distribution of bonded and nonbonded electron pairs. However, molecular structure is defined by the positions of the nuclei. The four pairs of electrons in both water and ammonia are tetrahedrally arranged around the central atom. Water, with only three atoms, is angular, and ammonia, with four atoms, is pyramidal (Figure 1.7). 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

Note In VSEPR theory the term bond pair is used for a single bond, a double bond, or a triple bond, even though a single bond consists of one pair of electrons, a double bond two pairs of electrons, and a triple bond three pairs of electrons. To avoid any confusion between the number of electron pairs actually involved in the bonding to a central atom, and the number of atoms bonded to that central atom, we shall occasionally use the term ligand" to indicate an atom or a group of atoms attached to the central atom. 

It is noteworthy that the to-bonded structure for ArF6 differs from that predicted by VSEPR theory. ArF6 is predicted to be of octahedral (Oh) symmetry, with three mutually perpendicular F i- Ar -h F triads and an s-type lone pair. In contrast, VSEPR predicts a pentagonal bipyramid (or other seven-vertex polyhedron) with some or all F-Ar-F angles less than 90°. The calculated equilibrium structure is in agreement with the co-bonding model. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

As can be seen from Table I, these weak CIF bonds occur only when the central atom has a coordination number in excess of 4 and possesses at least one free Cl valence electron pair. In addition to Gillespie s simple VSEPR theory, the following general rule has been proposed by Christe (53), which permits the prediction of whether, and how many, semi-ionic bonds are to be formed ... 

A) The carbon of carbon dioxide has two double bonds. Because there are no unshared pairs of electrons on the central carbon atom, VSEPR theory predicts a linear molecular geometry (type AX2). 

The basic principle underlying VSEPR theory is that valence electron pairs, whether they re lone pairs or they occur within bonds, prefer to be as far from one another as possible. There s no sense in crowding negative charges any more than necessary. 

In general, it is assumed in VSEPR theory that the repulsive interactions between bonding (b) and non-bonding or lone ( ) electron pairs are ordered ... 

One very important difference between VSEPR theory and MO theory should be noted. The MOs of the water molecule which participate in the bonding are three-centre orbitals. They are associated with all three atoms of the molecule. There are no localized electron pair bonds between pairs of atoms as used in the application of VSEPR theory. The existence of three-centre orbitals (and multi-centre orbitals in more complicated molecules) is not only more consistent with symmetry theory, it... 

The VSEPR theory assumes that the four electrons from the valence shell of the carbon atom plus the valency electrons from the four hydrogen atoms form four identical electron pairs which, at minimum repulsion, give the observed tetrahedral shape. To rationalize the tetrahedral disposition of four bond-pair orbitals with those of the 2s and three 2p atomic orbitals of the carbon atom, sp3 hybridization is invoked. 

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