VSEPR

10 Key excerpts on "VSEPR"

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  eBook - ePub

  Inorganic Chemistry

  Some New Facets

  • Ram Charitra Maurya(Author)
  • 2021(Publication Date)
  • De Gruyter

    (Publisher)

  Chapter I  Valence shell electron pair repulsion (VSEPR) theory: principles and applications

  1.1  Introduction

  This theory was first formulated by Sidgwick and Powell (1940) based on the repulsions between electron pairs, known as valence shell electron pair repulsion (VSEPR) theory to explain molecular shapes and bond angles of molecules of non-transition elements. Later on Gillespie and Nyholm (1957) developed an extensive rationale (basis/underlying principle) called VSEPR model of molecular geometry.

  According to this theory, the shape of a given species (molecule or ion) depends on the number and nature of electron pairs surrounding the central atom of the species.

  1.2  Postulates of VSEPR theory: Sidgwick and Powell

  The various postulates of this theory are as follows:

  1. The unpaired electrons in the valence shell of central atom form bond pairs (bps) with surrounding atoms while paired electrons remain as lone pairs (lps).
  2. The electron pairs surrounding the central atom repel each other. Consequently, they stay as far apart as possible in space to attain stability.
  3. The geometry and shape of the molecule depend upon the number of electron pairs (bond pair as well as lone pair) around the central atom.
  4. The geometrical arrangements of electron pairs with different number of electron pairs around central atom are given in Table 1.1 .

  Table 1.1: Shapes of the various molecules depending upon the number of shared electrons around the central metal atom.

  1.3  Rules proposed by Gillespie and Nyholm

  The following rules have been proposed by Ronald Gillespie and Ronald Sydney Nyholm of University College of London to explain the shape of a number of polyatomic molecules or ions.

  1.3.1

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  eBook - ePub

  The VSEPR Model of Molecular Geometry

  • Ronald J Gillespie, Istvan Hargittai, Istvan Hargittai(Authors)
  • 2013(Publication Date)
  • Dover Publications

    (Publisher)

  Although it is only a rough approximation to assume that electron-pair domains are nonoverlapping, we will see in Chapter 7 that the overlapping of electron-pair domains in a valence shell is minimized in the most stable arrangement in which the electron pairs are as far apart as possible. In any other arrangement, there is more overlapping of the electron-pair domains and the energy of the system is increased. Thus electron pairs behave as if they repel each other and this is the reason for the name VSEPR (valence-shell electron-pair repulsion) model. We will see that in discussing molecular geometry it will generally be more convenient to emphasize the space-occupying properties of electron pairs rather than their mutual repulsion. Moreover, the original emphasis on electron-pair repulsion led to the erroneous idea, sometimes found in discussions of the VSEPR model, that it is a classical electrostatic model and therefore not in accord with the quantum mechanical description of a molecule. The discussion in this book is therefore based mainly on the effect of the different sizes and shapes of electron-pair domains on molecular geometry. An alternative name for the model would be the VSEPD (valence-shell electron-pair domain) model, but we will continue to use the name VSEPR because it is now so well established.

  The “spheres and elastic bands” model described above gives the arrangement of a given number of equal spheres in which they are packed as closely as possible around a central point. An alternative way to obtain these same arrangements is to consider the packing of a given number of equal circles, or circular domes, on the surface of a sphere so that they occupy as much as possible of the surface of the sphere. The central points of each circular dome then have the same arrangement as obtained by the points-on-a-sphere model (Figure 3.8 ).

  Figure 3.8 The octahedral arrangement of six equal circles on a sphere maximizes the area covered by the circles.

  DEVIATIONS FROM IDEAL BOND ANGLES

  We have seen that we can predict the general shapes of molecules from the total number of electron pairs in the valence shell of the central atom. However, the bond angles in many molecules are not exactly equal to the ideal angles corresponding to these shapes. Qualitative predictions of these deviations from the ideal bond angles can be made by taking into account the differences in the sizes and shapes of the electron-pair domains in a valence shell. The electron-pair domains in a valence shell are not all equivalent for three important reasons:

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  Introduction to Molecular Science

  • Ageetha Vanamudan(Author)
  • 2023(Publication Date)
  • Delve Publishing

    (Publisher)

  Nuclear repulsions and nuclear attractions are the two categories that may be used to classify these interactions with one another. Depending on how you look at it, these interactions can be put into either the category of nuclear repulsions or nuclear attractions. The VSEPR theory is built on several fundamental presumptions, some of which are listed below for your perusal: The electrons in the valence shell are less attracted to one another than the other pairs of electrons. This is because the electron clouds in the valence shell are negatively charged. In comparison to the other pairs of electrons, this one is unique. These electrons make an effort to occupy space in a manner that will maintain the distance between them as short as possible. This is done so that they would not be repulsed by one another when they come into contact with one another. Studying the elements that make up a molecule of gaseous beryllium fluoride can provide a particularly illustrative example of how the VSEPR theory operates. The Lewis structure of the molecule of beryllium fluoride reveals that the beryllium atom at the nucleus of the molecule is only surrounded by two electron pairs. This can be seen by looking at the molecule. Because there won’t be any lone electron pairs present when there are two bonds present, the core atom won’t have any lone electron pairs because there won’t be any lone electron pairs present (Raies& Bajic, 2016). Another benefit of this arrangement is that it reduces the amount Molecular Structures 139 of electrostatic repulsion that occurs between electrons when they are on opposite sides of the core atom rather than when they are together. Once the acute angle reaches 180 degrees, a connection has been made between the two points. The molecular structure of an atom and the geometry of its electron pairs are two distinct concepts that are relevant here.

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  General Chemistry: Atoms First

  • Young, William Vining, Roberta Day, Beatrice Botch(Authors)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  For example, numerous experiments have shown that water, H 2 O , has a bent (nonlinear) shape. The Lewis structure, however, can be drawn to show a linear arrangement of atoms. H O H The valence shell electron-pair repulsion (VSEPR) theory allows chemists to easily predict the shapes of molecules and ions made up of nonmetals. According to VSEPR theory: ● ● Positions around a central atom are occupied by structural electron pairs , non-bonding or bonding electrons that repel one another and are arranged so as to avoid one another as best as possible. ● ● Structural electron pairs can be nonbonding electrons, where each pair of electrons (or single electrons, in free radicals) is counted as one structural electron pair on a central atom. ● ● Structural electron pairs can be bonding electrons, where each bond (single or multiple) is counted as one structural electron pair on a central atom. ● ● The electron-pair geometry is the arrangement of the structural electron pairs around the central atom. ● ● The shape (also called molecular geometry ) is the arrangement of atoms around the central atom. The electron-pair geometry is defined by the arrangement of structural electron pairs around the central atom. In large molecules with multiple central atoms, we will describe the electron-pair geometry around each central atom. When a central atom is surrounded by two, three, four, five, or six structural pairs of electrons, they are arranged in one of the following ideal electron-pair geometries (Interactive Table 7.1.1). Interactive Table 7.1.1 also shows the bond angles that are characteristic for each ideal electron-pair geometry. Bond angle is the angle formed by the nuclei of two atoms with a central atom at the vertex.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  This example illustrates that VSEPR is just a first approximation. It is just a model, incomplete and flawed (as most simple models are), but it is nevertheless useful, because it can be used to predict the molecular geometry of most small molecules with reasonable accuracy. Thus far, we have seen three different molecular shapes: tetrahedral, trigonal pyramidal, and bent. We will now explore other common molecular shapes that are accurately predicted with VSEPR theory. Trigonal Planar Geometry Consider the structure of BF 3 . Boron has three valence electrons, each of which is used to form a σ bond. Therefore, the boron atom requires the use of three hybridized orbitals (rather than four, as we have seen in previous examples). Applying valence bond theory, we expect the boron atom to be sp 2 hybridized (with three equivalent sp 2 -hybridized orbitals and one empty p orbital). As we saw in Section 1.10, sp 2 hybridization is associated with trigonal planar geometry, in which all bond angles are 120° (Figure 1.37). The term “trigonal” indicates that the boron atom is connected to three other atoms, and the term “planar” indicates that all atoms lie in the same plane. FIGURE 1.37 BF 3 has trigonal planar geometry, with bond angles of 120°. B F F F F F F B 120° 120° 120° Once again, the VSEPR model correctly predicts the trigonal planar geometry of BF 3 . There are three electron pairs that are repelling each other (steric number = 3), and they are expected to posi- tion themselves in space so as to achieve maximal separation. This is only accomplished in a trigonal planar arrangement, with bond angles of 120°, exactly as observed. Linear Geometry Consider the structure of BeH 2 . Beryllium has two valence electrons, each of which is used to form a σ bond. The beryllium atom therefore requires only two hybridized orbitals, and must be sp hybrid- ized.

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  Principles of Inorganic Chemistry

  • Brian W. Pfennig(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  The larger size of the S atom in H 2 S further exag- gerates this effect, since the H─S single bond is also longer than the O─H single bond, allowing the bonding pairs in H 2 S to lie farther from the central atom than those in H 2 O. 104.5° 92.1° H H S H H O Despite its relative simplicity, the VSEPR model is an incredibly powerful one. The theory is ultimately based on the Pauli principle, which leads to an apparent repulsion of pairs of electrons occupying differently sized domains around the central atom. VSEPR theory can not only predict the molecular geometry of most small molecules, but it can also make qualitative comparisons between bond angles and bond lengths and determine which ligand will occupy which site in a trigonal bipyramidal electron geometry. However, the theory does have its limitations. For example, the best Lewis structure for Li 2 O predicts a bent molecular geometry analogous to the isoelec- tronic H 2 O molecule even though the experimentally determined geometry for lithium oxide is linear. One reasonable argument is that the bonding in Li 2 O is more ionic in nature than it is covalent, and therefore the VSEPR model no longer applies. A second possible reason is that the lithium nuclei are so large compared to the central O atom that there is a steric (ligand–ligand) repulsion between the two Li atoms, which forces the molecule into a linear molecular geometry. 182 5 MOLECULAR GEOMETRY The ligand closest packing model, which is developed in the next section, can bet- ter predict this geometry. Another exception occurs for the BrF 6 - ion, which has seven electron domains. VSEPR theory would predict this ion to have a pentag- onal bipyramidal electron geometry with significant bond angle deviations due to the larger size of the lone pair domain. Infrared spectroscopy of BrF 6 - , however, indicates that the molecular geometry is very close to that of a perfect octahe- dron.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Next, identify the arrangement of the electron pairs in 3D space. According to VSEPR theory, we presume that all electron pairs will be positioned so as to achieve maximal dis‑ tance from one another. Since there are four electron pairs, VSEPR theory predicts a tetrahe‑ dral arrangement of electron pairs. In this case, a lone pair is predicted to occupy one corner of the tetrahedron, giving rise to trigonal pyramidal geometry (just like NH 3 ). 1.27 Use VSEPR theory to predict the geometry for each of the following structures: (a) H B H H H ⊝ (b) B F F F (c) ⊕ H N H H H (d) Cl Cl C H Cl 1.28 Compare the structures of a carbocation and a carbanion: Carbanion Carbocation C ⊕ C ⊝ In one of these ions, the central carbon atom is trigonal planar, while the other is trigonal pyramidal. Using VSEPR theory, assign the correct geometry to each ion. 1.29 Ammonia (NH 3 ) will react with a strong acid, such as hydronium (H 3 O + ), to give an ammonium ion, as shown below. This type of process is an acid‑base reaction, which will be the topic of Chapter 3. Using VSEPR theory, determine whether you expect a change in bond angles when ammonia is converted into an ammonium ion. Explain. Ammonia N H H H Ammonium ion ⊕ N H H H H ⊕ H 3 O H 2 O + + 1.30 When sand is coated with a layer of trimethylhydroxysilane, (CH 3 ) 3 SiOH, it repels water and can no longer get wet. Hydrophobic sand (aka, magic sand) is fun to play with, but it can also have useful applications in agriculture to reduce water consumption. 8 Predict the geometry for the silicon atom in trimethylhydroxysilane. Try Problems 1.40, 1.41, 1.50, 1.55, 1.56, 1.58 SKILLBUILDER LEARN the skill 1.8 PREDICTING GEOMETRY STEP 3 Identify the geometry. STEP 1 Determine the steric number. PRACTICE the skill APPLY the skill need more PRACTICE? 1.11 Dipole Moments and Molecular Polarity Recall that induction is caused by the presence of an electronegative atom, as we saw earlier in the case of chloromethane.

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  Chemistry

  The Molecular Nature of Matter

  • Neil D. Jespersen, Alison Hyslop(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  9.4 | Valence Bond Theory So far we have described the bonding in molecules using Lewis structures and the shapes using the VSEPR model. Lewis structures, however, tell us nothing about why covalent bonds are formed or how electrons manage to be shared between atoms. Nor does the VSEPR model explain why electrons group themselves into domains as they do. Thus, we must look beyond these simple models to understand more fully the covalent bond and the factors that determine molecular geometry. There are two theories of covalent bonding that have evolved based on quantum theory: the valence bond theory (or VB theory, for short) and the molecular orbital theory (MO theory). They differ principally in the way they construct a theoretical model of the bond- ing in a molecule. The valence bond theory imagines individual atoms, each with its own orbitals and electrons, coming together to form the covalent bonds of the molecule. The molecular orbital theory doesn’t concern itself with how the molecule is formed. It just views a molecule as a collection of positively charged nuclei surrounded in some way by electrons that occupy a set of molecular orbitals, in much the same way that the electrons in an atom occupy atomic orbitals. (In a sense, MO theory would look at an atom orbital as if it were a special case—a molecule having only one positive center, instead of many.) Bond Formation by Orbital Overlap According to VB theory, a bond between two atoms is formed when two electrons with their spins paired are shared by two overlapping atomic orbitals, one orbital from each of the atoms joined by the bond. By overlap of orbitals we mean that portions of two atomic orbitals from different atoms share the same space. 2 An important part of the theory, as suggested by the italic type above, is that only one pair of electrons, with paired spins, can be shared by two overlapping orbitals.

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  Chemistry

  An Atoms First Approach

  • Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste, , Steven Zumdahl, Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste(Authors)
  • 2020(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  If the molecule is symmetrical, any polar bonds will cancel resulting in a nonpolar molecule overall. In Chapter 9 we will discuss intermolecular forces resulting from polar molecules and how the strength of these forces influence molecular properties such as boiling point and surface tension of liquids. CONNECTING TO ATOMS 4.1 Polar Molecules—It’s All About Symmetry Molecule Lewis Structure Electron Pair Arrangement Electrostatic Potential Diagram CH 4 C H H H H H H C H H H H C H H NH 3 N H H H H H N H Lone pair H H N H H 2 O O H H O Lone pair Lone pair H H H O H 155 4.1 Molecular Structure: The VSEPR Model Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. The VSEPR Model—How Well Does It Work? The VSEPR model is very simple. There are only a few rules to remember, yet the model correctly predicts the molecular structures of most molecules formed from non- metallic elements. Molecules of any size can be treated by applying the VSEPR model to each appropriate atom (those bonded to at least two other atoms) in the molecule. Thus we can use this model to predict the structures of molecules with hundreds of atoms. It does, however, fail in a few instances. For example, phosphine (PH 3 ), which has a Lewis structure analogous to that of ammonia, would be predicted to have a molecular structure similar to that for NH 3 , with bond angles of approximately 107 degrees. However, the bond angles of phosphine are actu- ally 94 degrees. There are ways of explaining this structure, but more rules have to be added to the model.

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  Chemistry

  The Molecular Nature of Matter

  • Neil D. Jespersen, Alison Hyslop(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  418 Theories of Bonding and Structure CHAPTER 9 CHAPTER OUTLINE 9.1 Five Basic Molecular Geometries 9.2 Molecular Shapes and the Valence Shell Electron Pair Repulsion (VSEPR) Model 9.3 Molecular Structure and Dipole Moments 9.4 Valence Bond Theory 9.5 Hybrid Orbitals and Molecular Geometry 9.6 Hybrid Orbitals and Multiple Bonds 9.7 Molecular Orbital Theory Basics 9.8 Delocalized Molecular Orbitals 9.9 Bonding in Solids 9.10 Bonding of the Allotropes of the Elements This Chapter in Context Solar panels require semiconductors to convert light energy into electric- ity. Our current technology relies on pure silicon for the semiconduc- tors because the bonding structure of silicon allows the electrons to flow under the right circumstances. In this image, silicon is being purified by a process called zone refining in which a rod of silicon is heated in zones from one end until it melts in that area and then the zone of heating is slowly moved up and down the rod. The impurities are moved to the ends of the rod as it is melted and solidified in zones. In this chapter we will explore the topic of molecular geometry and study theoretical models that allow us to explain, and in some cases pre- dict, the shapes of small molecules. We will also examine theories that explain, in terms of wave mechanics and the electronic structures of atoms, how covalent bonds form and why they are so highly directional in nature. The knowledge gained here will be helpful in later chapters in this book when we examine the molecular basis of physical properties such as melting points and boiling points. These same principles will be extended to larger and larger molecules in your future studies of organic chemistry and biochemistry. We will also discuss how the bonding of bulk solids such as silicon imparts specific properties to solids.

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