Which of the following best describes the orbital overlap in the given molecule? (I) p to p, (II) sp to s, (III) sp to sp. (A) I only, (B) II only, (C) I and II only, (D) II and III only, or (E) I, II, and III.
When we visualize a bond between atoms within a molecule, we might think of a cartoon like this, where the atoms participating in the bond are sharing their electrons between them. But this cartoon is actually far too simplistic. Chemical bonds result from the overlap of atomic orbitals. For example, in the H2 molecule, each of the hydrogen atoms in the molecule have one electron in their outer 1s orbital. When these two atomic orbitals move closer to each other and overlap, this creates the bond. There’s two kinds of overlap that create the bonds within molecules. One is the end-to-end overlap of orbitals, like we’ve just seen with the hydrogen molecule. The other is the side-by-side overlap of orbitals, usually this kind of overlap is between p orbitals.
When orbitals overlap end to end, it creates what’s called a 𝜎 bond, often indicated by the Greek lowercase letter 𝜎. 𝜎 bonds are responsible for single bonds within a molecule. When orbitals overlap side by side, the resulting bond is called a 𝜋 bond, indicated by the Greek lowercase letter 𝜋. 𝜋 bonds are responsible for multiple bonds, double or triple bonds, within a molecule. So if we had a molecule like this, for example, where one atom was bonded with a single bond and the other two were bonded with a double bond. The single bond and one of the bonds in the double bond would be 𝜎 bonds. And the other bond within the double bond would be a 𝜋 bond. So we can identify what type of bond that we’re going to have in the molecule for this question.
The hydrogens and carbons are bonded with a single bond, so that means each of those would be 𝜎 bonds. The carbons are bonded together with a triple bond, so one of those will be a 𝜎 bond and the other two will be 𝜋 bonds. Before we decide what kinds of overlap are in the given molecule for this question, we need to know one more thing about orbital overlap. To do that, let’s take a look at the orbital overlaps within the CH4 molecule which is called methane. The hydrogens in methane each have one electron in their outer 1s orbital. The carbon and methane has two electrons in its outer 2s orbital and two electrons in 2p orbitals. So we’ll have hydrogen’s s atomic orbitals and carbon’s s and p atomic orbitals to create the structure for methane.
If we try to create methane’s structure using these orbitals, we’ll quickly realize that all of the bonds won’t have the same type of orbitals overlapping. But all the bonds in methane should be the same. This is because carbon doesn’t actually use its s and p orbitals to create bonds. All of its orbitals get mixed together to create four identical sp3 hybrid orbitals, which visually look somewhat similar to both an s orbital and a p orbital. With these new hybrid orbitals, carbon can create four identical single bonds to form the methane structure.
So with all this in mind, let’s take a look at the molecule in this question to figure out which kinds of orbital overlap the molecule will have in its structure. As we’ve discussed, the hydrogen will have its electrons within its outer 1s atomic orbital and that the carbon’s outer electrons will be in the 2s and 2p orbitals. Each carbon will need to create two 𝜎 bonds, so there’s going to be some degree of hybridization between the atomic orbitals of carbon here. But this hybridization is going to be different than what we saw in the methane molecule. This time, carbon will hybridize so that it has two identical sp orbitals and two p orbitals still remaining.
One of the sp orbitals from each of the carbons will overlap to create the single bond between the two carbons. This describes the type of overlap in statement (III), sp to sp. The other sp orbital from each carbon will overlap with hydrogen’s s orbital to create the 𝜎 bond between the hydrogen and the carbon. This describes the type of orbital overlap in statement (II), sp to s. The remaining two p orbitals for each carbon will then overlap to create the two 𝜋 bonds between them, which describes the type of overlap in statement (I), p to p. So of the types of orbital overlap in this question, all three, p to p; sp to s; and sp to sp, were present in the molecule that we’re given, which matches answer choice (E).