For statements I and II, state for each if they are true or false. I) The structure of NO three minus can be shown using more than one structural formula. II) NO three minus is stabilized by the delocalization of electrons, as illustrated by its various resonance structures. If both are true, state if II is a correct explanation for I.
The first part of this question is asking us about the structural formula of NO three minus, which is a polyatomic anion called nitrate. To determine the structural formula of nitrate, we’ll have to draw its Lewis structure. So let’s start with that. Our first step will be to determine the number of valence electrons that will be in our structure, which we can do using the periodic table. Nitrogen has five valence electrons. Oxygen has six valence electrons. And then we have one extra electron because nitrate is a negatively charged ion. So this means the nitrate ion has 24 valence electrons.
Now, let’s create our structure. The first thing we should do is place the atoms and connect them with single bonds. Nitrogen goes in the center because it has fewer valence electrons. So it can bond to more elements. Since each single bond contains two electrons, we’ve used up six of our total valence electrons so far. So we have 18 valence electrons remaining to place in our structure. If we place these 18 valence electrons in our structure, we’ll run out of electrons before nitrogen has a full octet. To remedy this, we can remove one of the lone pairs from oxygen and use it to create a double bond between the oxygen and the nitrogen.
Now, all of the atoms in our structure have full octets. So our structure is complete. When I was drawing the structure, I arbitrarily chose the oxygen on the left to create a double bond. But I could’ve chosen any of the three oxygens in the nitrate ion to form a double bond between. Statement I says that the structure of nitrate can be shown using more than one structural formula. We’ve just shown that this statement is true. We can place the double bond between the nitrogen and any of the three oxygens in nitrate. So there’s a total of three structural formulas that we can draw for nitrate.
So what do these three Lewis structures that we’ve drawn tell us about the real structure of the nitrate ion? Does the position of the double bond move between the oxygen atoms instantaneously? Do some nitrate ions have the double bond in one position, whilst others have the double bond in another? Well, how we’ve drawn our structure, we’d expect that one of the bonds in the structure would be shorter than the others because it’s a double bond. But as it turns out, the bonds between the nitrogen and each oxygens are all identical. They each have a bond length of 1.28 angstroms, which is 10 to the minus 10 meters. So what’s actually going on here with the structure of nitrate?
These three structures that we’ve drawn for the nitrate ion are called resonance structures. Which are structures that have the same placement of atoms. But they differ in the location of their bonds and their lone pairs. It’s important to recognize that resonance structures do not depict the real structure for the molecular ion. The position of the double bond doesn’t move back and forth. And some molecules or ions don’t have one structure or another. So then what does the real structure of nitrate look like?
Well, it turns out the real structure is kind of like a hybrid between all of the resonance structures that we can draw. There is a single bond between the nitrogen and its oxygen, as well as a partial bond between each of them. This partial bond is due to the delocalization of electrons. When we have a single or a double bond, the electrons are localized in one spot. But when the electrons are delocalized, they get spread out throughout the whole structure. Because we have delocalized electrons spread out throughout the structure in the nitrate ion. The structure is more stable than we would expect it to be if the nitrate ion had one of the three resonance structures that we drew above.
Now, let’s take a look at statement II. Which says that the nitrate ion is stabilized by the delocalization of electrons, as illustrated by its various resonance structures. As we’ve just discussed, this is true. Though we can draw three resonance structures for the nitrate ion. The real structure is a hybrid between the three of them, which has delocalized electrons. Resulting in partial bonds between the nitrogen and each of the oxygens, which stabilizes the overall structure.
Since statement I and II are both true, we should state if II is a correct explanation for I. So we need to determine if the nitrate ion being stabilized by delocalization of electrons is the reason that the structure can be shown using more than one structural formula. This is true. The delocalization of electrons in the nitrate ion is precisely the reason that we can draw more than one structural formula for it. Because each of the structural formulas that we can draw are one of the resonance structures for the nitrate ion.