Electron-dot model of bonding

Electron-Dot Model of Bonding Lewis Structures CHAPTER 1... 

Fig. 11 is a drawing of a two-dimensional analogue of the electron-domain model of ethane. Large circles represent valence-shell electron-domains (superimposed on them are the valence strokes of classical structural theory). Plus signs represent protons of the "C—H bonds. The nuclei of the two carbon atoms are represented by small dots in the trigonal interstices of the electron-pair lattice. While these nuclei would not necessarily be in the centers of their interstices, exactly, it can be asserted that an (alchemical) insertion of the two protons on the... 

Eortunately, modern quantum chemistry provides good approximate solutions to the Schrodinger equation and also, perhaps more importantly, new qualitative concepts that we can use to represent and understand chemical bonds, molecular structure, and chemical reactivity. The quantum description of the chemical bond is a dramatic advance over the electron dot model, and it forms the basis for all modern studies in structural chemistry. 

We began our definitions of acids and bases with the Arrhenius model. We then saw how the Br0nsted-Lowry model, by introducing the concept of a proton donor and proton acceptor, expanded the range of substances that we consider acids and bases. We now introduce a third model, which further broadens the range of substances that we can consider acids. This third model is the Lewis model, named after G. N. Lewis, the American chemist who devised the electron-dot representation of chemical bonding (Section 9.1). While the Br0nsted-Lowry model focuses on the transfer of a proton, the Lewis model focuses on the transfer of an electron pair. Consider the simple acid-base reaction between the ion and NH3, shown here with Lewis structures ... 

Figure 3. Structural representation of the computational models A-F. Models B, D, and E are combined QM/MM models where the regions enclosed in the dotted polygons represent the QM region. The regions outside the polygons are treated by a molecular mechanics force field. For the electronic structure calculation of the QM region, the covalent bonds that traverse the QM/MM boundary (the dotted polygon), have been capped with hydrogen atoms. In model A the atoms labelled 1 through 4 are the atoms that have been fixed in the calculations of models A through E.

In Fig. 2.3 the dots represent the atoms in the Holstein model, and (a) shows the situation where an electron sets up a bond between two atoms, pulling them together. A physical example is a hole in the valence band of a solid rare gas (e.g. Xe), forming a molecule Xe . 

In the next section, we explore how the shell model can be used to explain periodic trends. An even further simplified shell model, known as an electron-dot structure, is then developed in Chapter 6 to assist you in understanding chemical bonding. As you use these models, please keep in mind that electrons are not really confined to the surface of one shell or another. Instead, any... 

The electron-dot structures described in Sections 7.6 and 7.7 provide a simple way to predict the distribution of valence electrons in a molecule, and the VSEPR model discussed in Section 7.9 provides a simple way to predict molecular shapes. Neither model, however, says anything about the detailed electronic nature of covalent bonds. To describe bonding, a quantum mechanical model called valence bond theory has been developed. 

The idealized polyacetylene chain of CH groups connected to one another by bonds of identical length and each having one pz electron (represented in Figure 10.1 by black dots) can serve as the simplest one-dimensional model of metal [6, 8-10]. In such system because of the exchange interaction... 

Fig. 10.1. One-dimensional metal model of interacting 2/ z-electrons in polymer chain from C-C- bonds of equal length [9] (a) the configurations of chain with repeat union (a). 2/jz-electrons are indicated by black dots (b) energy band scheme for 2/ z-electrons EF Fermi energy.

Lewis structure electron dot structure dot structure. A model pioneered by Gilbert N. Lewis and Irving Langmuir that represents the electronic structure of a molecule by writing the valence electrons of atoms as dots. Pairs of dots (or lines) wedged between atoms represent bonds dots drawn elsewhere represent nonbonding electrons. 

Here is an example in which the MO model has a distinct advantage over the Lewis dot picture. B2 is found only in the gas phase solid boron is found in several very hard forms with complex bonding, primarily involving B12 icosahedra. B2 is paramagnetic. This behavior can be explained if its two highest energy electrons occupy separate it orbitals as shown. The Lewis dot model cannot account for the paramagnetic behavior of this molecule. 

The HOMO of NH3 is slightly bonding, because it contains an electron pair in an orbital resulting from interaction of the 2p orbital of nitrogen with the H orbitals of the hydrogens (from the zero-node group orbital). This is the lone pair of the electron-dot and VSEPR models. It is also the pair donated by ammonia when it functions as a Lewis base (discussed in Chapter 6). 

Both ionic and covalent bonds involve valence electrons, the electrons in the outermost energy level of an atom. In 1920, G. N. Lewis, the American chemist shown in Figure 9, came up with a system to represent the valence electrons of an atom. This system—known as electron-dot diagrams or Lewis structures —uses dots to represent valence electrons. Lewis s system is a valuable model for covalent bonding. However, these diagrams do not show the actual locations of the valence electrons. They are models that help you to keep track of valence electrons. 

Lewis structure (p. 243) A model that uses electron-dot structures to show how electrons are arranged in molecules. Pairs of dots or lines represent bonding pairs. 

This chapter provides a substantial introduction to molecular structure by coupling experimental observation with interpretation through simple classical models. Today, the tools of classical bonding theory—covalent bonds, ionic bonds, polar covalent bonds, electronegativity, Lewis electron dot diagrams, and VSEPR Theory—have all been explained by quantum mechanics. It is a matter of taste whether to present the classical theory first and then gain deeper insight from the... 

Models can help you visualize the three-dimensional structures of molecules. Consider the simplest molecule that exists—hydrogen, H2. Two hydrogen atoms share a pair of electrons in a nonpolar covalent bond, as shown in the electron dot structure. 

Whether you use gumdrops, electron dot diagrams, or supercomputers, the ability to model bonding between atoms is useful. By determining the shape and polarity of a molecule, you can predict its behavior and properties. In Chapter 10, you ll learn more about the forces between particles and the effects they have on the physical states of substances. 

Before turning to the two bonding models, let s discuss a method for depicting the valence electrons of interacting atoms. In the Lewis electron-dot symbol (named for the American chemist G. N. Lewis), the element symbol represents the nucleus and inner electrons, and the surrounding dots represent the valence electrons (Figure 9.4). Note that the pattern of dots is the same for elements within a group. 

Whenever Lewis appfied his model to covalent compounds, he noted that the atoms seemed to share pairs of electrons. He also noted that most compotmds contained even numbers of electrons, which suggested that electrons exist in pairs. He therefore replaced his cubic model of the atom, in which eight electrons were oriented toward the surfaces of a cube, with a model based on pairs of electrons. In this notation, each atom is surrounded by up to fotu" pairs of dots, corresponding to the eight possible valence electrons. This symbohsm is still in use today. The only significant modification is the use of fines to indicate covalent bonds formed by the sharing of a pair of electrons. The Lewis structures for F2 and O2 are written as follows ... 

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