Which of the following species would display dipole–dipole attractions? A) Ar, B) O₂, C) CH₄, D) Cl⁻, or E) CO.
Before we press on with the question, we should just check we know the names of all the candidates. Ar is argon. O₂ is molecular oxygen. CH₄ is methane. Cl⁻ is the chloride ion. And CO is carbon monoxide. The question asks us, which of these five options would display dipole–dipole attractions?
Dipole–dipole attractions are instances where permanent dipoles are attracted to one another, 𝛿-positive to 𝛿-negative. This phrase doesn’t generally refer to things like London dispersion forces, where instantaneous dipoles interact. Instead, we’re looking for permanent dipoles.
The first candidate, argon, exists as an atomic gas under most circumstances and therefore cannot be polar. An atom of argon consists of a positively charged nucleus surrounded by a negatively charged electron cloud. But the electron cloud is evenly distributed around space. So while you could draw little dipoles coming out from the nucleus, they would all cancel out. Therefore, argon cannot be a correct answer because there’s no way for it to display dipole–dipole attractions.
What about a diatomic like molecular oxygen? The two oxygen atoms are completely identical. And the electron cloud is evenly distributed. This means, like with the atom, there’s no overall dipole. The other way you can think about this is that oxygen atoms have the same electronegativity. So no one of them is going to pull electrons in one direction over the other. So oxygen cannot be our correct answer.
So what about methane? Methane is a little bit more complicated. The electronegativity of carbon is 2.55, while that for hydrogen is 2.20. So the carbon atom, being more electronegative, is going to pull more than its fair share of electron density towards it, away from the hydrogen. This makes the carbon 𝛿-negative and the hydrogens 𝛿-positive. This gives four individual dipoles, one for each bond. However, each hydrogen is identical and each bond is identical. So because of the symmetry, all the dipoles cancel out. Therefore, methane has no permanent dipole. Since it can’t be polar, it can’t exhibit dipole–dipole attractions.
What about the case with a simple ion like Cl⁻? Just like with argon, we have a positively charged nucleus surrounded by an evenly distributed electron cloud. While the ion is overall negative, you could draw exactly the same dipoles in all directions like you could with argon. Since all these dipoles would cancel each other out, there wouldn’t be a permanent dipole. And therefore, we would not see dipole–dipole attractions. Instead, with a simple ion like Cl⁻, we would expect ionic attractions, not dipole–dipole attractions. We’d have to have a counter ion with a positive charge to see that. Otherwise, we’d see ionic repulsions.
Our last candidate is carbon monoxide, CO. The electronegativity of carbon as we said before is 2.55, but that for oxygen is much greater, at 3.44. Therefore, the oxygen is going to draw some of that electron density towards itself. So the oxygen becomes 𝛿-negative and the carbon becomes 𝛿-positive. This gives us the permanent dipole we’ve been searching for. This permanent dipole of carbon monoxide will give us those dipole–dipole attractions. The 𝛿-positive sections of one carbon monoxide molecule will be attracted to the 𝛿-negative portions of another. Therefore, of the five species given, the only one that would display dipole–dipole attractions would be CO.