CHEMISTRY COMMUNITY

What are your best methods for finding intermolecular forces of a molecule? Do we start out with drawing the Lewis structure?

I would start out drawing the Lewis structure because that will determine whether or not a molecule is polar and capable of a hydrogen bond. If you draw it out and it is not symmetrical you know it is polar, and if it has an OH, NH, or HF bond than you know it is capable of a hydrogen bond. If anyone else has other approaches please include them.

I second what the comment above says. Further, you should also determine if the question is asking to determine the IMF's the molecule can have with another molecule of itself or another molecule that the problem specifies. If you have a OH, HN, or HF bond you know there is possibility for H-bonding. If the dipole moments do not cancel or the shape is asymmetrical the molecule has possibility for dipole-dipole interactions. Dispersion depends on polarizability, but every molecule has it.

The part that confused me what the induced-dipole induced dipole IMFs. From what I remember from discussion section, the larger the non-polar molecule, the easier it is to disperse the charges within the molecule, making the IMFs stronger, compared to if you had shorter non-polar molecules interacting.

Personally, I would prefer to draw the Lewis structure first to see whether the molecule is ionic (ionic-ionic, ionic-dipole) -->whether polar (ionic-dipole, dipole dipole, dipole-induced dipole), or nonpolar (dipole-induced dipole, induced dipole-induced dipole), with the help of either net dipole moment or VSEPR model. Even though, in the molecule, several same atoms are attached to the same central atom, the molecule could actually still potentially be polar, due to lone pair electrons.

It's good to start off drawing the Lewis Structure. From there you can determine whether the molecule is polar or nonpolar and capable of forming a hydrogen. If a molecule is polar then there is a dipole-dipole interaction. All molecule tend to have a induced-induced interaction.

I think the best way is to look at the formula first. If it is definitely polar and does not have symmetry, you can probably assume that there are dipole forces. But of course, drawing a lewis structure would be safer. So draw the structure and see if there is symmetry when you are not sure.

Its also always important to remember that sometimes even if it looks symmetrical, such as with tetrahedrals, you still need to look at the dipole moments to determine polarity.

You should start with Lewis structures and then measure the dipole moments to see if it is really nonpolar.

I start out by drawing a Lewis structure and determining whether the compound is polar. If it's polar, it will experience dipole-dipole interactions. If it has an H bonded to an F, O, or N, it experiences H-bonding. If it's non polar, it only experiences London Dispersion Forces.

I would also start out by drawing the molecule and determining whether it is polar or nonpolar. From there, you can tell if the molecule has LDFs and Dipole Dipole IMFs if the molecule is polar or LDFs only if it's nonpolar.

Jasmine 3L wrote:What are your best methods for finding intermolecular forces of a molecule? Do we start out with drawing the Lewis structure?

yes, drawing the Lewis structure is the best first step, because it will help you best visualize, as much as possible, what is happening regarding which atoms are attached to which atoms. if any H atoms are attached to N, O, or F its a hydrogen bond. In addition, look at the shape; this will also tell you a lot: if the atoms are all the same surrounding the central atom it's most likely non-polar (no dipole-dipole) if there are some lone pairs there may be a net dipole making the molecule polar (dipole-dipole).