Keeping Track of Bonding Lewis Structures

There are strong forces between the different strands of polymers in Teflon, so they tend to form single chains that pack closely together. 

Other polymers with other elements bound to carbon tend to have weaker forces between strands and are often more branched as depicted here. 

We ve now surveyed some important aspects of chemical bonding, and we ve gained an understanding of a number of key concepts. But if we want to think about the connections between bonding and the properties of compounds, we often need to know not just that bonds have formed, but how many bonds have formed and between which elements. The Lewis structures we introduced in Section 7.3 provide a powerful tool for doing this. In drawing Lewis structures for 

Although it may seem routine after a little practice, it is always vitally important to carry out Step 1 of this process. Using the wrong number of valence electrons Is among the most common errors that students make in drawing Lewis structures. 

As we introduce our algorithm for drawing Lewis strucmres, it will help if we use a simple molecule to illustrate each step. In keeping with our fluorine theme, we ll choose oxygen difluoride (OF2) as our first example. 

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In this chapter, you will use Lewis structures often to represent molecules and the simplest formula unit of an ionic solid. Drawing a Lewis structure for a molecule lets you see exactly how many electrons are involved in each bond, and helps you to keep track of the number of valence electrons. In the example below, notice that there are two ways to show the bonding pairs of electrons. Some chemists use dots only. 

These two-dimensional, formula-like diagrams help you count and keep track of valence electrons, and communicate essential information about the placement and bonding of atoms or ions in a molecule. Chemists often draw Lewis structures in a way that suggests the shape of a molecule. However, this is not their function. It is important to remember that Lewis structures do not communicate any information about a molecule s shape. To represent the shapes of real molecules, you need a model that depicts them in three-dimensions. 

Lewis structure shows the connectivity between atoms in a molecule by a number of dots equal to the number of electrons in the outer shell of an atom of that molecule. A pair of electrons is represented by two dots, or a dash. When drawing Lewis structures, it is essential to keep track of the number of electrons available to form bonds and the location of the electrons. The number of valence electrons of an atom can be obtained from the periodic table because it is equal to the group number of the atom. Eor example, hydrogen (H) in Group lA has one valence electron, carbon (C) in Group 4A has four valence electrons, and fluorine (E) in Group 7A has seven valence electrons. 

In this reaction, a proton is transferred from the acid to the base. The unshared pair of electrons on the base is used to form the new bond to the proton while the electrons of the H—A bond remain with A as an unshared pair. Previously, curved arrows have been used to show electron reorganization in resonance structures. Organic chemists also use these arrows to show electron movement in reactions. The arrows are a kind of bookkeeping device that helps us keep track of electrons as the Lewis structures of the reactants are converted to the Lewis structures of the products. Remember, an arrow always points from where the electrons are to where they are going. It does not point from where the hydrogen (or other atom) is to where it is going. 

Lewis structures are helpful for keeping track of electron transfers in bonding and for making sure that the octet rule is obeyed. As well,... 

Lewis structures are partieulariy usc-rul because they make electron bookkeeping possible and act as reminders of the number of valence electrons present. Simpler, hn-wever, 1 the use of Kckule structures, or Hn bond structures, in which two-electron covalent bond is indicated as a line drawn between atoms. Lone pairs of nonbonding valence electrons are often not shown when drawing lane-bond structures, though it s still necessary to keep track of them mentally. Some examples ure shown inT ble 1.2. 

We will use one of the two Lewis structures and not the hybrid in drawing benzene, because it is easier to keep track of the electron pairs in the n bonds (the 7i electrons). 

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. 

Before we explore the problem space for a simple proton transfer reaction, we need to understand the basics of bonding and define a consistent nomenclature. In order to use the electron flow paths, you first need to be able to keep track of atoms and electrons— write Lewis structures correctly and easily. 

Electron flow paths are written in the language of Lewis dot structures and curved arrows. Lewis dot structures are used to keep track of all electrons, and curved arrows are used to symbolize electron movement. You must be able to draw a proper Lewis structure complete with formal charges accurately and quickly. Your command of curved arrows must also be automatic. These two points cannot be overemphasized, since all explanations of reactions will be expressed in the language of Lewis structures and curved arrows. A Lewis structure contains the proper number of electrons, the correct distribution of those electrons over the atoms, and the correct formal charge. We will show all valence electrons lone pairs are shown as darkened dots and bonds by lines. 

In this chapter we have explored the structure of organic compounds. This is important since structure determines reactivity. We have seen that weak bonds are a source of reactivity. Strong bonds are made by good overlap of similar-sized orbitals (same row on periodic table). Bends or twists that decrease orbital overlap weaken bonds. Lewis structures and resonance forms along with electron flow arrows allow us to keep track of electrons and explain the changes that occur in reactions. VSEPR will help us predict the shape of molecules. Next we must review how bonds are made and broken, and what makes reactions favorable. Critical concepts and skills from this chapter are ... 

The octet rule predicts that atoms form enough covalent bonds to surround themselves with eight elechons each. When one atom in a covalently bonded pair donates two electrons to the bond, the Lewis structure can include the formal charge on each atom as a means of keeping track of the valence electrons. There are exceptions to the octet rule, particularly for covalent beryllium compounds, elements in Group 3A, and elements in the third period and beyond in the periodic table. 

Perhaps you will not be surprised, then, you to learn that an even more general model of acids and bases was proposed by American chemist G. N. Lewis (1875-1946). Recall that Lewis developed the electron-pair theory of chemical bonding and introduced Lewis structures to keep track of the electrons in atoms and molecules. He applied his electron-pair theory of chemical bonding to acid-base reactions. Lewis proposed that an acid is an ion or molecule with a vacant atomic orbital that can accept (share) an electron pair. A base is an ion or molecule with a lone electron pair that it can donate (share). According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Note that the Lewis model includes all the substances classified as Bronsted-Lowry acids and bases and many more. 

Organic chemistry involves reactions between organic compounds and other organic or inorganic species. These reactions can involve both bond-breaking and bond-forming processes, and the key to both is the movement of electrons. Lewis structures provide the bookkeeping system to help us keep track of electrons in reactions. 

Figure 7.6 I The Lewis symbols for elements in the first three periods of the periodic table are shown. Notice the similarities among elements in the same group. Lewis structures help keep track of electrons involved in chemical bonds.

When one atom in a covalently bonded pair donates two electrons to the bond, the Lewis structure can include the formal charge on each atom as a means of keeping track of the valence electrons. 

By the 1920s, vitalism had been discarded. Chemists were aware of constitutional isomerism and had developed the structural theory of matter. The electron had been discovered and identified as the source of bonding, and Lewis structures were used to keep track of shared and unshared electrons. But the understanding of electrons was about to change dramatically. 

Formal charges do nnt represent actual charges on atoms in a molecule. In the O3 molecule, for example, there is no evidence that the central atom bears a net -1-1 charge or that one of tlie terminal atoms bears a — 1 charge. Assigning formal charges to the atoms in the Lewis structure merely helps us keep track of the electrons involved in bonding in the molecule. 

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