Chemistry
Understanding Intermolecular Forces
Quick fact
Even the weakest type of intermolecular force, the London dispersion force, is responsible for holding atoms together in liquid helium near absolute zero.
Why this is interesting
Water and oil don’t mix, but why? And why does a gecko stick to a ceiling? The secret is a hidden world of attractions between molecules.
Read the full explanation
Understanding Understanding Intermolecular Forces
Imagine molecules as tiny magnets, but with different strengths. Some molecules have permanent positive and negative ends (poles) because of how their electrons are distributed. Others, even nonpolar ones, can temporarily create poles when electrons shift. These temporary or permanent poles cause molecules to attract each other. These attractions are called intermolecular forces. They are weaker than the covalent bonds that hold atoms together inside a molecule, but they are crucial. It’s like the difference between the strong glue that builds a brick (chemical bond) and the weaker forces that stack the bricks together (intermolecular force). The strength of these forces determines whether a substance is a gas, liquid, or solid at room temperature, and whether it evaporates quickly or boils at a high temperature.
A deeper explanation
The underlying cause of all intermolecular forces is the distribution of electrons in molecules. Electrons are always in motion, so even in nonpolar molecules they can be temporarily unevenly distributed, creating an instantaneous dipole. This dipole can induce a dipole in a neighboring molecule, resulting in a weak attraction called a London dispersion force; larger molecules have more electrons, so these forces are stronger. In polar molecules, the permanent separation of charge (due to electronegativity differences) creates dipole-dipole forces, which are stronger than dispersion. A special, exceptionally strong type of dipole-dipole interaction is hydrogen bonding, which occurs when a hydrogen atom is bonded to a highly electronegative atom (F, O, or N) because the hydrogen becomes so positively polarized that it strongly attracts lone pairs on neighboring molecules. The cumulative effects of these forces explain why water has a relatively high boiling point, why oils are less dense and don’t mix with water, and why the gecko’s toe hairs can use van der Waals forces to cling to surfaces.