Lewis structures repulsion theory

In Chapter 7, we used valence bond theory to explain bonding in molecules. It accounts, at least qualitatively, for the stability of the covalent bond in terms of the overlap of atomic orbitals. By invoking hybridization, valence bond theory can account for the molecular geometries predicted by electron-pair repulsion. Where Lewis structures are inadequate, as in S02, the concept of resonance allows us to explain the observed properties. 

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 ... 

The shapes of molecules are determined by actual experiments, not by theoretical considerations. But we do not want to have to memorize the shape of each molecule. Instead, we would like to be able to look at a Lewis structure and predict the shape of the molecule. Several models enable us to do this. One of the easiest to use is valence shell electron pair repulsion theory, which is often referred to by its acronym VSEPR (pronounced vesper ). As the name implies, the theory states that pairs of electrons in the valence shell repel each other and try to stay as far apart as possible. You probably remember this theory from your general chemistry class. The parts of VSEPR theory that... 

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

Obviously, the formulas CO2 and SO2 do not provide any information about the shapes of these molecules. However, there is a model that can be used to predict the shape of a molecule. This model is based on the valence shell electron pair repulsion (VSEPR) theory. Using this model, you can predict the shape of a molecule by examining the Lewis structure of the molecule. 

E2.7 The Lewis structure of ICV and its geometry based on VSEPR theory is shown below. The central iodine atom is sunounded by six bonding and one lone pair. However, this lone pair in the case of ICU" is a slereochemically inert lone electron pair. This is because of the size of iodine atom—large size of this atom spread around all seven electron pairs resulting in a minimum repulsion between electron pairs. As a consequence all bond angles are expected to be equal to 90° with overall octahedral geometry. 

Valence-bond theory, 32—34, 42, 46 Valence electrons, 10 and Lewis structures, 20 Valence-shell electron pair repulsion and molecular geometry, 26-29, 45 L-Valine, 1054, 1059... 

Background Covalent bonding occurs when atoms share valence electrons. In the Valence Shell Electron Pair Repulsion (VSEPR) theory, the way in which valence electrons of bonding atoms are positioned is the basis for predicting a molecule s shape. This method of visualizing shape is also based on the molecule s Lewis structure. 

If we can draw Lewis structures for covalent molecules, we can predict their shapes by applying a few simple rules. The theory that accounts for the shapes of molecules is called valence shell electron pair repulsion theory (VSEPR theory) and is based on the reasoning that electron pairs attempt to get as far away from other electron pairs as possible because their negative charges repel each other. 

Valence shell electron pair repulsion theory (VSEPR) can be used to predict the shapes of molecules. According to this theory, the geometry of a molecule is such that the valence-electron pairs of the central atom are kept farthest apart to minimize the electron repulsions. Again, you have to view molecules in terms of Lewis structure so that the shape of the molecules can be predicted with the VSEPR theory. 

Lewis theory, in combination with valence shell electron pair repulsion (VSEPR) theory, can be used to predict the shapes of molecules. VSEPR theory is based on the idea that electron groups—lone pairs, single bonds, or multiple bonds—repel each other. This repulsion between the negative charges of electron groups on the central atom determines the geometry of the molecule. For example, consider CO2, which has the Lewis structure ... 

In Section 4.3, we learned that the shape of a molecule is an important factor in determining the properties of the substances that it composes. For example, we learned that water would boil away at room temperature if it had a straight shape instead of a bent one. We now develop a simple model called valence shell electron pair repulsion (VSEPR) theory that allows us to predict the shapes of molecules from their Lewis structures. 

The shapes of molecules and ions can be predicted by the valence shell electron pair repulsion theory (VSEPR). If the Lewis structure is drawn for a molecule or a polyatomic ion, the shape of this molecule or ion can be predicted using this theory. 

The original VSEPR theory (Figure 4.56) is a classical theory that accounts for the shapes of most main group molecules. Lewis structures are simple representations of molecules and ions that emphasize the importance of electron pairs (represented by dots and/or crosses). VSEPR theory proposes that the favoured molecular shape is that which minimizes the repulsion between electron pairs in the valence shell of the central atom. 

Lewis structures of all but the simplest molecules do not show the shape of the molecule. A collection of rules known as valence-shell electron repulsion theory (VSEPR theory), in which regions of electron density (attached atoms and lone pairs) are supposed to adopt positions that minimize their repulsions, is often a helpful guide to the local shape at an atom, such as the tetrahedral arrangement of single bonds around a carbon atom. This theory should also be familiar from introductory chemistry courses. 

In Section 1.4, we represented the electronic structures of molecules as Lewis structures, which enable us to explain the connectivity of atoms in a molecule. This is the most fundamental feature of molecular structure. We then used valence-shell electron-pair repulsion (VSEPR) theory to explain molecular geometry. 

Lewis s original theory could not take into account the shape adopted by molecules. Gillespie and Nyhohn (1957) developed the currently accepted modem theory of chemical bond formation (MO and VB theories), which uses the valence-shell electron pair repulsion model (VSEPR) to account for molecular structure (Gillespie, 1970). VSEPR states that molecular shape is caused by repulsions between electron pairs in the valence shell. 

Linnett s procedure may be viewed as a refinement of classical structural theory In Linnett s theory, the van t Hoff-Lewis tetrahedral model is applied twice, once to each set of spins, with the assumption that, owing to coulombic repulsions between electrons of opposite spin, there may be, in some instances, a relatively large degree of spatial anticoincidence between a system s two spin-sets. 

---