Formal charges in resonance structures

PROBLEMS For each drawing, try to draw the curved arrows that get you from one drawing to the next. In many cases you will need to draw more than one arrow. 

In this example, we can see that one of the lone pairs on the oxygen is coming down to form a bond, and the C=C double bond is being pushed to form a lone pair on a carbon atom. When both arrows are pushed at the same time, we are not violating either of the two commandments. So, let s focus on how to draw the resonance structure. Since we know what arrows mean, it is easy to follow the arrows. We just get rid of one lone pair on the oxygen, place a double bond between the carbon and oxygen, get rid of the carbon-carbon double bond, and place a lone pair on the carbon  

This is why we need resonance—it shows us where there are regions of high and low electron density. If we draw resonance structures without formal charges, then what is the point in drawing the resonance structures at all  

Another way to assign formal charges is to read the arrows properly. Let s look at our example again  

EXERCISE 2.20 Draw the resonance structure that you get when you push the arrows shown below. Be sure to include formal charges. 

The electrons are not really moving. We are just treating them as if they were. 

In Section 1.4, we learned how to calculate formal charges. Resonance structures very often contain formal charges, and it is absolutely critical to draw them properly. Consider the following example  

Draw the resonance structure below. Be sure to include formal charges. 

Carefully read what the curved arrows indicate. 

It is absolutely critical to draw these formal charges. Structures drawn without them are wrong. In fact, if you forget to draw the formal charges, then you are missing the whole point of resonance. Let s see why. Look at the resonance structure we just 

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SkillBuilder 2.7 Assigning Formal Charges in Resonance Structures... 

Both O and N are bonded to C.) Which atom has the negative formal charge in your structure What is the shape of the ion Show a resonance structure for this ion. 

Two resonance structures must have the same number of electrons (and atoms, for that matter). The formal charges in both structures must add up to the same number. 

Resonance, (a) Write two contributing structures consistent with the octet rule for hydrazoic acid, HN3, for which the sequence of bonded atoms is HNNN. Keep formal charges to a minimum, (b) Locate all formal charges in each structure, (c) Which if any is the correct formula of the molecule Explain, (d) Predict the two N-to-N bond distances for HN3 relative to single-, double-, and triple-bond distances. 

Thus, the nitrite can be represented by two equivalent resonance contributors. Note that the nitrogen atom has no formal charge in either structure. The single-bonded oxygen atom in each case has a formal minus charge. 

In this chapter, procedures for drawing molecular structures have been illustrated, and a brief overview of structural inorganic chemistry has been presented. The structures shown include a variety of types, but many others could have been included. The objective is to provide an introduction and review to the topics of VSEPR, hybrid orbitals, formal charge, and resonance. The principles discussed and types of structures shown will be seen later to apply to the structures of many other species. 

The boron atoms in resonance structures A and B possess a formal negative charge. The simplest representation for these compounds is... 

Different Lewis structures do not in general make the same contribution to the resonance structure. It is possible to decide which structures are likely to make the major contribution by examining how the electrons are shared in each one and assigning a formal charge to each atom. The lower the formal charges for a structure, the greater its contribution to a resonance hybrid. 

The relative stability of resonance structures can be judged by the same rules that were previously introduced to judge the stability of Lewis structures the octet rule, the number and location of formal charges, and the interactions between charges in the structure. 

The sulfate ion provides another example of the value of formal charges in explaining resonance. When the structure is drawn as if the S-0 bonds were single bonds, it is seen that there is a +2 formal charge on the sulfur atom ... 

Type-1 systems (heteroatom in position 1) possess a formally charge-free resonance formula such as 2a. The cyclic structure can be represented only by the charged formulas la, b. This is the reason the more stable species in this case is the open-ring form (2). 

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