How to Deduce Intermolecular Force for Simple Molecule
Here is the recommended sequence to deduce the intermolecular force (IMF) of a simple molecule:
1. Is H-F, H-O or H-N bond present?
If yes, its IMF will be hydrogen bonds. If not, consider the next point.
2. Are there any polar bonds?
A bond is polar when an electronegative element (F, O, N, Halogens, S) is bonded to a non-electronegative element.
If no, its IMF will be instantaneous dipole - induced dipole (id-id) attractions.
Examples of non-polar molecules with non-polar bonds are elements and hydrocarbons.
If yes, consider the next point.
3. Consider the shape of molecule. Do the dipole moments cancel?
If the dipole moments do not cancel out exactly, the molecule is polar and IMF will be permanent dipole - permanent dipole (pd-pd) attraction.
If the dipole moments cancel out exactly, the molecule is non-polar hence IMF will be id-id attraction.
Sometimes it can be difficult to visualise the dipole moments and deduce if they cancel out or not.
Another way is to memorise the shapes that have high symmetry, the dipole moments will cancel out exactly and therefore the molecule is non-polar.
The shapes with high symmetry are basic shapes (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral) and square planar.
Topic: Intermolecular Forces, Physical Chemistry, A Level Chemistry, Singapore
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