Which statement is correct? A) N2 has one sigma bond and two pi bonds. B) CCl4 has two sigma bonds and two pi bonds. C) HC CH has three sigma bonds and four pi bonds. D) NaCl has one sigma bond and no pi bonds. Or E) the molecule HC C CH3 has five sigma bonds and two pi bonds.
When atoms bond covalently, we often say that they’re sharing electrons. And one electron is contributed by each atom that’s participating in the bond. This picture of bonding is a rather simplified picture of what’s really going on. Let’s take a look at a molecule of H2. Each hydrogen atom has just one electron in its 1s orbital. If each of the atomic orbitals from each hydrogen atom move closer to each other and overlap, that overlap between the orbitals forms a bond. Now, we have a covalently bonded H2 molecule. If orbitals overlap end to end, like we just saw in the example with the hydrogen molecule, we call the resulting bond a sigma bond. All sigma bonds are single bonds.
Orbitals can also overlap side by side to form bonds. When this happens, the bond that forms is called a pi bond. Pi bonds form double or triple bonds between atoms in a molecule. You often see sigma bonds denoted by the lowercase Greek letter 𝜎 and pi bonds denoted by the lowercase Greek letter 𝜋. Let’s take a look at the molecule ethene as an example.
There is a single bond between each of the hydrogens and the carbon. Each of these would be a sigma bond caused by the end-to-end overlap of orbitals. One of the bonds between the carbon and the carbon would also be a sigma bond. But the other one would be a pi bond. So let’s take a look through our answer choices to figure out how many sigma and pi bonds each of our answers has. We’ll need to look at the structure for each of our answer choices to figure this out.
Our first answer choice is N2. So let’s draw the structure for N2. Each nitrogen has five valence electrons. So we have 10 valence electrons to place in the structure. Connecting our atoms with a single bond uses up two of these electrons. If we place our remaining eight electrons, we’ll run out of electrons before each nitrogen has a full outer shell. We can remedy this by removing one of the lone pairs from one of the nitrogens and using it to create a double bond. If we do this again, we’ll arrive at the correct structure for N2.
If we look at our structure, we have three bonds in between the nitrogens. This means that nitrogen should have one sigma bond caused by the end-to-end overlap of orbitals and two pi bonds caused by the side-to-side overlap of orbitals. This is what answer choice A says, that nitrogen has one sigma bond and two pi bonds. So this must be the answer choice that we’re looking for. But let’s take a look through the rest of our answer choices, just so we understand.
Our next answer choice is CCl4. There are four single bonds in this structure. Since they’re all single bonds, they must be sigma bonds. So there are four sigma bonds in CCl4. So CCl4 has four sigma bonds not two sigma bonds and no pi bonds.
Our next molecule is called ethyne. It has three sigma bonds, two for the single bonds between the carbons and hydrogens and one between the carbons. The remaining two bonds, that are the double and the triple bond in between the carbons, are pi bonds. So ethyne has three sigma bonds and two pi bonds, not four pi bonds.
Our next answer choice is NaCl. The bond between the sodium and the chlorine in sodium chloride is not a covalent bond but an ionic bond. Ionic bonds are caused by the electrostatic attraction between oppositely charged ions. They’re not caused by orbital overlap. So sodium chloride has neither sigma bonds nor pi bonds.
Our last answer choice is the molecule 1-propyne. There are five single bonds in the structure, each of which would be a sigma bond. And then, there’s also a sigma bond in between the carbon and the carbon that is triple bonded. The remaining two bonds between the triple-bonded carbons would be pi bonds. So 1-propyne has six, not five, sigma bonds and two pi bonds.
So of our answer choices, it was true that nitrogen has one sigma bond and two pi bonds.