Resonance Structures (Simplified) - Online Tutor, Practice Problems & Exam Prep

Resonance structures are multiple valid Lewis dot representations for polyatomic ions with pi bonds, such as the nitrite anion (NO₂^-). These structures illustrate the movement of electrons from pi bonds or lone pairs, showing that the actual structure is a resonance hybrid, an average of all possible forms. Dotted lines indicate the presence of pi bonds in the hybrid. Understanding resonance is crucial for grasping molecular stability and reactivity in organic chemistry.

Resonance Structures (Simplified) Concept 1

Resonance structures are sets of two or more valid Lewis dot structures for polyatomic species possessing at least one pi bond. In resonance structures, we observe the movement of only electrons from either a pi bond—recall that double bonds and triple bonds contain pi bonds—or a lone pair. Consider NO₂⁻, which is our nitrite anion. This can be depicted in one of two ways, where the oxygen on the left is double-bonded or the oxygen on the right is double-bonded. Since both configurations are possibilities, you could show both, and they are considered resonance structures. There are two resonance structures for the nitrite ion.

Notice we use double-sided arrows. These arrows are used to indicate that the resonance structures are equivalent to each other, equating one with another. The real structure is represented by the composite of these structures, often referred to as the resonance hybrid. This means that the real structure isn't exclusively resonance structure 1 or 2, but rather an average of the two.

The resonance hybrid is a composite of all significant resonance structures. To draw the resonance hybrid, we place a dotted line wherever a pi bond has been. For instance, there was a pi bond on the oxygen on the left and one on the right. Thus, we draw a dotted line at these locations to illustrate this fact. We can still place the structure in brackets and denote the charge on the outside, saying that this could represent your resonance hybrid.

Resonance Structures (Simplified) Example 1

Determine the remaining resonance structures possible for the carbonate ion, which is CO₃^2-. Alright. So remember, when we talk about our resonance structures, we're just showing the different places a pi bond could exist within the structure. So, one possible resonance structure is instead of having the oxygen on the top being double bonded, let's do the one on the left being double bonded. Notice also when the oxygen is double bonded, it doesn't have 3 lone pairs; it only has 2. It is the ones that are single bonded that have 3 lone pairs attached to them. And because it's an ion, you have to put it in brackets with the charge on the top right corner. But let's say that it's not the oxygen on the left or the top that is double bonded, but instead, it's the one on the right. So, this would be yet another way of drawing the carbonate ion. Again, the ones that are single bonded have 3 lone pairs. The oxygen that is double bonded has 2 lone pairs. Put it in brackets and the charge on the outside. So these 2 would be the other resonance structures that exist for the carbonate ion. So, the carbonate ion has 3 different resonance structures. Again, the resonance hybrid itself would just be a composite or average of these 3. Here, it's not asking for it, but just realize that in this particular case, it wouldn't be any of the 3 being the real structure, it'd be a blending or an average of all 3.

Determine the remaining resonance structures possible for the phosphate ion, PO₄^3–.

Determine the remaining resonance structures possible for the phosphate ion, which is PO43-. So, remember here, we're just putting the pi bonds in other places, giving us different options. In the first image, we have the pi bond being here on the top oxygen, but remember there's an equal chance of it being on the right side, the left side, or the bottom oxygen. So we just have to show those other possibilities. Don't forget the lone pairs. Remember, oxygens that are single bonded have 3 lone pairs, while oxygens that are double bonded only have 2. It's an ion, so put it in brackets with the charge in the top right corner. So this would be a resonance structure. Let's say, instead of the oxygen on the right being double bonded, it's the oxygen on the bottom that's double bonded. Make sure you place the right number of electrons around the oxygen and the phosphorus. It still has its one lone pair. You don't have to put it in the same position every time for phosphorus as long as it's on the phosphorus, that's all that matters. And then finally, the last possibility is maybe it is the oxygen on the left that was double bonded, and the others are single bonded. Brackets and then the charge in the top right corner. Alright. So here, these would be the other three resonance structures that are possible for the phosphate ion.

Here’s what students ask on this topic:

What are resonance structures in chemistry?

Resonance structures are multiple valid Lewis dot representations for a polyatomic ion or molecule that has at least one pi bond. These structures illustrate the movement of electrons from pi bonds or lone pairs. For example, the nitrite anion (NO₂^-) can be represented with the double bond on either oxygen atom. The actual structure is a resonance hybrid, which is an average of all possible resonance forms, indicating the delocalization of electrons.

How do you determine the resonance structures of a molecule?

To determine the resonance structures of a molecule, follow these steps: 1) Draw the Lewis structure of the molecule. 2) Identify all possible locations for pi bonds and lone pairs. 3) Move electrons from pi bonds or lone pairs to form new structures, ensuring that the overall charge and number of electrons remain the same. 4) Use double-headed arrows to indicate that the structures are resonance forms. The actual structure is a resonance hybrid, an average of all resonance forms.

What is the significance of resonance structures in organic chemistry?

Resonance structures are significant in organic chemistry because they help explain the stability and reactivity of molecules. The resonance hybrid, which is the average of all resonance forms, represents the true structure of the molecule. This delocalization of electrons can lower the energy of the molecule, making it more stable. Understanding resonance is crucial for predicting how molecules will interact in chemical reactions.

How do you draw a resonance hybrid?

To draw a resonance hybrid, follow these steps: 1) Identify all major resonance structures of the molecule. 2) Draw a single structure that incorporates elements from all resonance forms. 3) Use dotted lines to indicate the presence of pi bonds that are delocalized across multiple atoms. 4) Place any formal charges outside the brackets. The resonance hybrid represents the average electron distribution of all resonance structures.

What is the difference between a resonance structure and a resonance hybrid?

A resonance structure is one of multiple valid Lewis dot representations of a molecule, showing different possible arrangements of pi bonds and lone pairs. A resonance hybrid, on the other hand, is the actual structure of the molecule, which is an average of all resonance structures. The resonance hybrid shows the delocalization of electrons and is more stable than any individual resonance structure.