Resonance Structures (Simplified): Study with Video Lessons, Practice Problems & Examples

Resonance structures are multiple valid Lewis dot structures for polyatomic ions with pi bonds, such as the nitrite ion (NO₂^-). These structures illustrate the movement of electrons from pi bonds or lone pairs, represented by double-sided arrows indicating their equivalence. The actual structure, known as the resonance hybrid, is a composite of these structures, depicted with dotted lines where pi bonds were. This concept emphasizes the importance of understanding electron distribution in molecular geometry and stability.

Resonance Structures (Simplified) Concept 1

Now, resonance structures are a set of two or more valid Lewis dot structures for polyatomic species possessing at least one pi bond. In resonance structures, there is the movement of only electrons from either a pi bond—remember, double bonds and triple bonds have pi bonds—or a lone pair. Here, we have NO₂⁻, which is our nitrite anion. It can be shown 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 are possibilities, you could show both, and they are resonance structures. Thus, there are two resonance structures for the nitride ion.

We use double-sided arrows to show that the resonance structures are equivalent to one another. The real structure is represented by the composite of the resonance structures called the resonance hybrid. The actual structure is neither resonance structure 1 nor 2; it is an average of the two. To draw the resonance hybrid, we place a dotted line wherever a pi bond has been observed. There was a pi bond on the oxygen on the left and another on the oxygen on the right. We draw a dotted line here and a dotted line there to show this fact. By putting it in brackets and writing the charge on the outside, we can denote 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's 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 resin structures possible for the phosphate ion, which is PO43-. So remember here, we're just putting the π bonds in other places, giving us different options. In the first image, we have the π 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; 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 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 three other 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 structures for a polyatomic ion or molecule that has at least one pi bond. These structures illustrate different possible distributions of electrons, particularly from pi bonds or lone pairs. They are represented using double-sided arrows to indicate their equivalence. The actual structure of the molecule or ion, known as the resonance hybrid, is a composite of these resonance structures, showing an average distribution of electrons. This concept helps in understanding the true electron distribution and stability of the molecule.

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 dot structure of the molecule. 2) Identify any pi bonds (double or triple bonds) and lone pairs of electrons. 3) Move electrons from pi bonds or lone pairs to form new bonds or lone pairs, ensuring that the overall charge and the number of electrons remain the same. 4) Draw all possible structures that result from these electron movements. 5) Use double-sided arrows to indicate that these structures are resonance forms of each other. The actual structure is a resonance hybrid, an average of these forms.

What is the significance of resonance structures in chemistry?

Resonance structures are significant because they provide a more accurate depiction of the electron distribution within a molecule or ion. They help explain the stability, reactivity, and physical properties of the molecule. The resonance hybrid, which is the average of all resonance structures, represents the true electron distribution. This concept is crucial for understanding molecular geometry, bond lengths, and the overall stability of the molecule, as it shows that electrons are delocalized rather than fixed in one position.

How do you draw the resonance hybrid of a molecule?

To draw the resonance hybrid of a molecule, follow these steps: 1) Identify all the resonance structures of the molecule. 2) Determine the locations of the pi bonds and lone pairs in each resonance structure. 3) Draw a single structure that includes all the atoms in the molecule. 4) Use dotted lines to represent the locations where pi bonds exist in any of the resonance structures. 5) Place any partial charges that result from the averaging of the resonance structures. This hybrid structure represents the true electron distribution in the molecule.

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

A resonance structure is one of multiple valid Lewis dot structures that represent different possible distributions of electrons in a molecule or ion with pi bonds. These structures are depicted using double-sided arrows to show their equivalence. In contrast, a resonance hybrid is the actual structure of the molecule, which is a composite or average of all the resonance structures. The resonance hybrid shows the true electron distribution, often represented with dotted lines where pi bonds exist in any of the resonance structures, indicating delocalized electrons.