Which of the following is a substance that exhibits a resonance structure? A) Structural isomer, B) octahedral crystal, C) sulphur dioxide, D) chloride salt, or E) amorphous.
A resonance structure is a set of two or more Lewis structures that collectively describe the structure of a chemical species. For example, let’s look at the resonance structures. For example, ozone or O3, which is the molecule present in Earth’s atmosphere responsible for shielding us from UV radiation, has two Louis structures that are equally valid. They differ based on where the double bond is located. These two Louis structures are resonance structures. This doesn’t mean that sometimes ozone has one Louis structure, and sometimes it has the other. Rather, the real structure for ozone is somewhere in between the two Lewis structures.
Both of the oxygen–oxygen bonds in ozone are the same length, which means they must be the same type of bond. The real structure for ozone contains a partial bond, which means that the electrons are delocalised or spread out throughout the structure. With all this in mind, let’s take a look at our answer choices to figure out which one will have a resonance structure.
Our first answer choice is structural isomers, which are molecules with the same molecular formula but with atoms connected in a different order. For example, this molecule on the left is called butane. And the molecule on the right is called 2-methyl propane. Both of them have the molecular formula C4H10 because they both have four carbons and 10 hydrogens. But their structure is different. So these two are structural isomers. These two structures are not two Lewis structures that are describing the same chemical species. So they aren’t resonance structures. So answer choice A is not correct.
An octahedral crystal is a crystal that’s formed from six atoms or compounds attached to a central metal atom. An example of this would be manganese(II) oxide. Octahedral crystals by definition are six things bonded to a central atom. So we wouldn’t expect there to be a collection of Lewis structures to describe their structure. So answer choice B is not correct.
Our next answer choice is sulphur dioxide or SO2. Let’s draw a Lewis structure for it to see if it has any resonance structures. Sulphur and oxygen both have six valence electrons. So sulphur dioxide has a total of 18 valence electrons. Sulphur will be our central atom here because it’s capable of forming more bonds than oxygen is. So we’ll place our atoms and connect them with single bonds. This has used up a total of four of our valence electrons, leaving us with 14 valence electrons to place in our structure. If we place these remaining 14 electrons, we’ll run out of valence electrons before all of our atoms have a full valence shell.
We can remedy this by removing one of the loan pairs on the oxygen to create a double bond between the sulphur and one of the oxygens. Now we have a complete Lewis structure. Unless you probably noticed, I arbitrarily picked the oxygen on the right to form a double bond. We could’ve drawn a Lewis structure that’s equally as valid that has the double bond on the oxygen on the left. So that makes these two structures resonance structures.
Just like ozone, the real structure for sulphur will be somewhere in between these two structures. So there will be a partial bond between the sulphurs and the oxygens. So sulphur dioxide does have resonance structures. This is the answer choice that we’re looking for. But let’s look through our other answer choices so we understand why they don’t have resonance structures.
A chloride salt will be a salt that’s formed from chlorine and some other metal element, for example, sodium chloride. These type of compounds exhibit ionic bonding. So they combine in specific ratios and form a repeating crystal structure called the lattice. When an ionic bond is formed, an electron is transferred from the metal to the nonmetal.
Resonance structures are used when the bonding in a molecule or ion can’t be described using a single Lewis structure. But in a chloride salt, we have ionic bonding, which is just a transfer of electrons from the metal to the nonmetal. We don’t need resonance structures to describe this.
Amorphous is a type of solid that has a varied structure instead of an ordered one. We can compare an amorphous solid to a crystalline solid. In a crystalline solid, the atoms are arranged in an orderly and repeating way, like fruit that’s neatly stacked in a display. And an amorphous solid, on the other hand, has no repeating order. The atoms are just all arranged randomly within the structure. An example of a crystalline solid would be our chloride salt that we just discussed. And an example of an amorphous solid would be glass. Amorphous describes a type of solid with a varied structure. It doesn’t describe a set of Lewis structures that are being used to describe the same chemical species. So E is not the answer we’re looking for.
Of our answer choices, answer choice C, sulphur dioxide, was the only one that exhibited a resonance structure.