Multiple Lewis structure with

Drawing Lewis Structures with Multiple Bonds... 

Species requiring multiple Lewis structures often involve electron delocalization with some electron pairs distributed over more than two atoms. 

You may notice that some Lewis structures with double bonds can be drawn multiple ways. For example, the molecule of ozone (O3) shown in Figure 6-5 can be drawn with the double bond on the left oxygen atom or the right oxygen atom. 

Chemists are still debating which of these two structures is dominant for P04 . Recent theoretical calculations based on quantum mechanics suggest to some researchers that the left structure is the dominant one. Other researchers claim that the bond lengths in the ion are more consistent with the right structure being dominant. This disagreement is a convenient reminder that, in general, multiple Lewis structures can contribute to the actual electron distribution in an atom or molecule. 

EXERCISE 8.7 Lewis Structure with a Multiple Bond... 

As shown in the output, this particular NBO search terminated successfully after only a single cycle, which satisfied the default search criteria. The search yielded a Lewis structure with one core (CR), one bond (BD), and three lone pair (LP) Lewis-type (L) NBOs, which described about 99.95% of the total electron density (i.e., 9.995 of the 10 electrons). These five L-type NBOs easily satisfied the default threshold (1.90e) for pair occupancy [the 0 under Low occ (L) ] and the remaining 17 non-Lewis (NL) NBOs were all well below the O.le occupancy threshold [ High occ (NL) ] to be considered a satisfactory Lewis structure. [The Dev entry refers to deviations from the initial guess that steers multiple cycles of the search algorithm (if required), beyond the scope of this book consult the NBO... 

Notice that the zinc atom is associated with only four valence electrons. Although this is less than an octet, the adjacent carbon atoms have no lone pairs available to form multiple bonds. In addition, the formal charge on the zinc atom is zero. Thus, Zn has only four electrons in the optimal Lewis structure of dimethyizinc. This Lewis stmcture shows two pairs of bonding electrons and no lone pairs on the inner atom, so Zn has a steric number of 2. Two pairs of electrons are kept farthest apart when they are arranged along a line. Thus, the C—Zn—C bond angle is 180°, and linear geometry exists around the zinc atom. 

Many of the Lewis structures in Chapter 9 and elsewhere in this book represent molecules that contain double bonds and triple bonds. From simple molecules such as ethylene and acetylene to complex biochemical compounds such as chlorophyll and plastoquinone, multiple bonds are abundant in chemistry. Double bonds and triple bonds can be described by extending the orbital overlap model of bonding. We begin with ethylene, a simple hydrocarbon with the formula C2 H4. 

Bond paths are observed between bonded atoms in a molecule and only between these atoms. They are usually consistent with the bonds as defined by the Lewis structure and by experiment. There are, however, differences. There is only a single bond path between atoms that are multiply bonded in a Lewis structure because the electron density is always a maximum along the internuclear axis even in a Lewis multiple bond. The value of pb does, however, increase with increasing Lewis bond order, as is shown by the values for ethane (0.249 au), ethene (0.356 au), and ethyne (0.427 au), which indicate, as expected, an increasing amount of electron density in the bonding region. 

Although such textbook diagrams are called Lewis structures, they are not the electron-dot diagrams that G. N. Lewis originally wrote for such species. Lewis s depiction of S042-, for example, is reproduced in Fig. 3.90. This shows a normal-valent S2+ ion with shared-pair bonds to four O- ions, which is fully consistent with the octet rule, with no intrinsic need for multiple resonance structures to account for the observed Td symmetry. According to Lewis s original concept, each ion is... 

Draw a single bond (one pair of electron dots or a line) between each pair of connected atoms. Place the remaining electrons around the atoms as unshared pairs. If every atom has an octet of electrons except H, He, Li, and Be, which are atoms with two electrons, the Lewis structure is complete. Shared electrons count towards both atoms. If there are too few electron pairs to do this, draw multiple bonds (two or three pairs of electron dots between the atoms) until an octet is around each atom (except H atoms with two). If there are two many electron pairs to complete the octets with single bonds then the octet rule Is broken for this compound. 

D. A conjugated molecule is a molecule with double bonds on adjacent atoms such as the molecule shown in A. Choice B and C give the definition of sigma and pi molecular orbitals. D is false because a resonance form is one of multiple equivalent Lewis structures, but these structures do not describe the actual state of the molecule. The anion will exist in a state between the two forms. 

Other examples of condensed structures with heteroatoms and carbon-carbon multiple bonds are given in Figure 1.5. You must learn how to convert a Lewis structure to a condensed structure, and vice versa. 

In many molecules, the choice of which atoms are connected by multiple bonds is arbitrary. When several choices exist, all of them should be drawn. For example, as shown in Figure 3-1, three drawings (resonance structures) of C03 are needed to show the double bond in each of the three possible C — O positions. In fact, experimental evidence shows that all the C — O bonds are identical, with bond lengths (129 pm) between double-bond and single-bond distances (116 pm and 143 pm respectively) none of the drawings alone is adequate to describe the molecular structure, which is a combination of all three, not an equilibrium between them. This is called resonance to signify that there is more than one possible way in which the valence electrons can be placed in a Lewis structure. Note that in resonance structures, such as those shown for in Figure 3-1, the electrons are drawn in different places but the atomic nuclei remain in fixed positions. 

Begin with a single pair of dots between each pair of bonded atoms. If no arrangement of single bonds provides a Lewis structure whose atoms satisfy the octet rule, the molecule might have multiple bonds. 

Write down the Lewis structure of the reactants, complete with formal charges, and draw any major resonance forms. Look for leaving groups, polarized single and multiple bonds, acids and bases. Classify into generic sources and sinks and then rank them. The reaction usually occurs between the best source and sink. Above all, note if the medium is acidic or basic. In basic media, find the best base, and then locate any acidic hydrogen within range (not more than 10 p Ta units above the pATabH of Ihe base). In acidic media, identify the best sites for protonation. Likewise, do not create a species that is more than 10 units more acidic than your acid. Understand what bonds have been made or broken, but do not lock into an arbitrary order as to which occurred first. 

Lewis and valenceA Lewis structure shows the valence electrons in a molecule. Two shared electrons form a structures single bond, with correspondingly more for multiple bonds. Some atoms may also have nonbonding electrons (lone-pairs). Valence structures show the bonds simply as lines. 

Step 5 (Skeleton 1) The structure above leaves the carbon atom without its octet. Because we are short two electrons for octets, we convert one lone pair to another bond. Converting the lone pair on the carbon to a multiple bond would still leave the carbon with only six electrons around it and would put ten electrons around the oxygen atom. Carbon and oxygen atoms always have eight electrons around them in a reasonable Lewis structure. Thus we instead try converting one of the lone pairs on the oxygen atom to another C-O bond. 

Resonance is possible whenever a Lewis structure has a multiple bond and an adjacent atom with at least one lone pair. The arrows in the following generalized structures show how you can think of the electrons shifting as one resonance structure changes to another. 

Lewis Structures for Molecules with Multiple Bonds Sometimes, you ll find that, after steps 1 to 4, there are not enough electrons for the central atom (or one of the central atoms) to attain an octet. This usually means that a multiple bond is present, and the following additional step is needed ... 

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