Notation Lewis structures

In each case, a plausible Lewis structure is given first, followed by the AX Em notation... 

The starting point for the Lewis structure is a notation for an atom and its valence electrons. The element symbol represents the core, that is, the nucleus and all the inner-shell electrons. The core carries a number of positive charges equal to the number of valence electrons. This positive charge is called the corechanie. Valence electrons are shown explicitly. For elements in the third and later rows of the periodic table, the d electrons in atoms of Main Groups III, IV, V, VI, and VII are counted as part of the core. Thus ... 

The Lewis structure notation is useful because it conveys the essential qualitative information about properties of chemical compounds. The main features of the chemical properties of the groups that make up organic molecules, ... 

Several kinds of formulas are used by organic chemists to represent organic compounds. Some of these formulas involve a shorthand notation that requires some explanation. Structural formulas actually show which atoms are bonded to which. There are two types of structural formulas, complete Lewis structures and condensed structural formulas. In addition, there are several ways of drawing condensed structural formulas. As we have seen, a Lewis structure symbolizes a bonding pair of electrons as a pair of dots or as a dash (—). Lone pairs of electrons are shown as pairs of dots. 

When a condensed structural formula is written for a compound containing double or triple bonds, the multiple bonds are often drawn as they would be in a Lewis structure. Table 1-3 shows examples of condensed structural formulas containing multiple bonds. Notice that the —CHO group of an aldehyde and the —COOH group of a carboxylic acid are actually bonded differently from what the condensed notation suggests. Condensed structures are assumed to follow the octet rule even if the condensed notation does not show the bonding. 

A. Structure, Bonding, and Nomenclature — Lewis structures, orbital hybridization, configuration and stereochemical notation, conformational analysis, systematic IUPAC nomenclature, spectroscopy (IR and 3H and 13 C NMR)... 

In this way, the valence shell configuration of the central atom, combined with the Lewis representation of the inert gas shell, gives a very useful way of visualising the distribution of the valence shell electrons in this chemical book-keeping exercise. In these Lewis structures all the electrons are equivalent and the dot or cross notation simply indicates the source of the electrons from the central atom or the terminal atoms. 

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 ... 

The Lewis structure of a compound can be arrived at by trial and error. We start by notating symbols that contain the correct number of valence electrons for the atoms in the molecule. We then pair electrons to indicate covalent bonds tmtil we come up with a Lewis structure in which... 

Whether you ve ever done Lewis structures before or not, follow the rules in Section 1-4 very closely. Become familiar with the number of electrons around common atoms and the common arrangements of these electrons in the bonds of molecules. This familiarity, brought about by doing lots of examples, is the best way to ensure that you will quickly and confidently be able to picture a Lewis structure for any of the types of species you will encounter later on. As you gain confidence through practice, you will be able to use shorthand notations, such as lines instead of dots for bonding electron pairs. 

Two important conventions involving arrows are introduced in Section 1-5. The first is the use of double-headed arrows between resonance forms. This is a special kind of notation because of the special role resonance forms play in organic chemistry. As shown in this section, many species have structures that cannot be represented by a single Lewis structure. They can only be described as intermediate in nature between two or more contributing forms, each of which by itself is an incomplete picture of the molecule s structure. We represent such a molecule by drawing the resonance forms separated by double-headed arrows and enclosed in brackets. The true structure is called the resonance hybrid. The only difference between the resonance forms is a different location for the electrons from one to the next. The same geometrical arrangement of the atoms is maintained in all the resonance forms. Caution molecules that actually exist as resonance hybrids are still often represented by only one Lewis structure. In cases like this you need to be aware of the fact that this is a shortcut used for convenience purposes only and that the real structure is still the resonance hybrid—the other resonance forms are implied even if they aren t written down. 

Note that the six-sided ring is shorthand notation for a benzene ring (—CgH5). Benzene is discussed in Section 14.5. Complete the Lewis structure for aspartame. How many C and N atoms exhibit sp hybridization How many C and O atoms exhibit sp hybridization How many cr and tt bonds are in aspartame Aspartame is an organic compound and the Lewis structure follows the guidelines outlined in Exercise 64. 

Traditionally used valence lines acquire new meaning they represent electron pairs. Within this semantics, the double bond is labeled by two lines which describe two electron pairs. Commonly, such notation is called the Lewis structure. 

Top) Two Lewis structures for the benzene ring. (Bottom) As a shorthand notation, the rings are usually represented without labeling the carbon and hydrogen atoms. 

To show the outermost electrons of an atom, we commonly use a representation called a Lewis structure, after the American chemist Gilbert N. Lewis (1875-1946), who devised this notation. A Lewis structure shows the symbol of the element, surrounded by a number of dots equal to the number of electrons in the outer shell of an atom of that element. In Lewis structures, the atomic symbol represents the nucleus and all filled inner shells. Table 1.3 shows Lewis structures for the first 18 elements of the Periodic Table. As you study the entries in the table, note that, with the exception of helium, the number of valence electrons of the element corresponds to the group number of the element in the Periodic Table for example, oxygen, with six valence electrons, is in Group 6A. 

Fig. 3 Representation of Lewis structures based on the attainment of closed shells by electron-pair sharing. The initial dot structures have been progressively replaced by line structures to represent the two-electron two-centre bonds. Dots are only retained when they have stereochemical consequences or are required to represent organic reactions using the curly arrow notation (see Fig. 4)...

Lewis structures could be used to represent organic reactions, and the resulting curly arrow representation, which may be viewed as an extension of the Sidgwick dative bond, is universally accepted and used to describe the mechanistic pathways of organic reactions. Figure 4 gives some specific examples of the notation as it is used in organic chemistry today. 

The formulas for the alkanes given in Table 25.1 are written in a notation called condensed structural formulas. This notation reveals the way in which atoms are bonded to one another but does not require drawing in all the bonds. For example, the Lewis structure and the condensed structural formulas for butane (C4H10) are... 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

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