Which of the following is the best description of an intermolecular force? A) An attractive or repulsive force between ions. B) An attractive force between molecules. C) A repulsive force between molecules. D) A covalent bond. Or E) an attractive or repulsive force between molecules.
In this question, we’re looking for the best description. This means a certain amount of judgement may be needed in order to evaluate one statement over another. To help us on our way, we can start by breaking down the word intermolecular. The prefix “inter” before intermolecular means between. So the whole word intermolecular can be read as between molecules. So an intermolecular force is some kind of force that acts in between molecules.
There are many types of intermolecular force — ionic, vandervells, polar. But not every substance exhibits every type of intermolecular force. So what we’re looking for is a statement that generally applies to all intermolecular forces. Let’s get cracking with option A, an attractive or repulsive force between ions. The first thing you might ask is, are ions molecules? Well, this is a bit of a shady area.
A substance like sodium acetate is said to have intermolecular ionic forces, while with something like sodium chloride, their ionic bonding would not necessarily be called intermolecular. For the purposes of this question, I am going to assume that there are ions that are considered molecules and therefore some ion–ion interactions that are considered intermolecular forces. However, not all intermolecular forces occur between ions.
So at the very best, we can class this as a poor description of an intermolecular force. Before I move on, I’m just gonna point out that attractive forces occur between ions of opposite charge, positive–negative, whereas repulsive ionic forces occur between ions of the same charge, positive–positive or negative–negative. So I’m going to class statement A as a poor description and move on to statement B.
An attractive force between molecules seems like a pretty good description of an intermolecular force. Lots of intermolecular forces are indeed attractive. But some are not, like positive–positive interactions and negative–negative interactions. Statement C, which says that intermolecular forces are a repulsive force between molecules, suffers from the same problem. Some intermolecular forces are repulsive, but some are not. Because of this, I’m going to class these two statements as poor descriptions of an intermolecular force and move on to option D.
A covalent bond is not an example of an intermolecular force at all. For instance, in hydrogen there is a covalent bond between the two hydrogen atoms. This is an example of an intramolecular force. “Intra” meaning inside, so intramolecular force meaning a force inside the molecule. I’m going to call this a terrible description of an intermolecular force. And therefore it is an incorrect answer.
Finally, we come to option E. Option E says that an intermolecular force is an attractive or repulsive force between molecules. This combines the best of both worlds of options B and C and makes up for their flaws. It’s a general statement which covers both attractive and repulsive cases. So of the five options given, this is the best description of an intermolecular force.