Getting Nerdy: Intermolecular Force

Atelectasis is bad. Our job as a clinician is to prevent and treat alveolar collapse. Alveolar collapse is caused by a mix of different mechanisms. For this article we’ll focus on:

Excessive fluid collection (obstructive)

lack of nitrogen within our lungs (non-obstructive)

Nitrogen acts as a big lazy gas that is poorly absorbed and just sits inside the alveoli to keep them expanded. But why do we need nitrogen to keep alveoli open? Why can’t the alveoli keep themselves open? It all comes down to some needy oxygen atom and two enabling hydrogen atoms on H2O.

We’ll start with looking at the forces at work between atoms and within a molecule before getting into the crazy relationship water has with itself. The intramolecular forces such as polar, non-polar, and covalent bonding occur between atoms. This is in contrast to intermolecular forces that hold molecules together in a solid or liquid. Intermolecular forces are generally much weaker than covalent bonds, but The Force is strong with hydrogen bonds.

There are three intermolecular forces that make our job as a clinician easier and harder:

Dipole-Dipole Interactions

London Dispersion Forces

Hydrogen Bonds

A dipole is a partial charge exhibited on portions of a molecule. This just means one end is positive and the other end is negative (like a battery). For instance, when looking at H2O, the oxygen is slightly negative because although it can’t accept another covalent bond, it still wants just a little bit more piece of the pie, which makes the hydrogen ions slightly positive.

A dipole-dipole force is sort of a gyration between attraction and repulsion between molecules. It’s a way for a mass of molecules in a solid or liquid to keep everyone happy with just the right amount of positive and negative.

London Dispersion Forces are near instantaneous changes in polarity due to hastily moving electrons. London Dispersion Forces are the weakest among the intermolecular forces because there is increased charge stability in the dipole-dipole force. This brings us to the culprit of our conundrum, the hydrogen bond.

The hydrogen bond is a very strong intermolecular force. Hydrogen bonding occurs with highly electronegative atoms which are oxygen, nitrogen, and fluorine. Remember that H2O has those partially negative and positive charges. Oxygen on H2O has two lone electron pairs. Electrons are by their nature are electronegative. Since hydrogen is a small, positive atom, it can get really close to the negative end of a nearby H2O molecule. This creates harmony among the collection of molecules, especially water. Water is like a pack of kids playing together, they’ll keep playing together until given a big reason to split up.

Hydrogen bonding creates a unique phenomenon when we look at the surface of a glass of water. H2O molecules turn hydrogen side up because hydrogen atoms are small and oxygen needs some more positivity in its' life. This creates a pretty positive charge at the surface of the water. The same applies for a water droplet. It wants to stick together to satisfy the negative oxygen and put the positive side out which allows the droplet to stick together. Water, when allowed, chooses to pull itself together unless it has something to disrupt the hydrogen bonds.

Our body produces surfactant which disrupts these surface level hydrogen bonds. It acts as an electromagnet mediator and breaks up all the surface level positivity allowing for a more electromagnetively satisfied mixture.

A decrease in surfactant will increase the surface tension within the alveoli due increased hydrogen bonding - this leads to atelectasis. The surfactant acts as a hydrogen bond disrupter to allow our alveoli to be nice and full of gas.

Someone might ask: “Well if both oxygen and nitrogen are electronegative why doesn’t nitrogen gas stick to the water within the alveoli causing collapse?” The point to remember is that intermolecular forces describe interaction between molecules in a solid or liquid, not between gases and liquids. Nitrogen gas is also a very stable gas and is completely okay with being it’s big bad self.

I hope this was a quick, nerdy way to utilize foundational knowledge that is applicable to being an excellent clinician. I’ve got a few of these in my back pocket so tweet me @vamedic if you’d like to get really nerdy about a topic. Stay classy and get nerdy.

-Adam LaChappelle (@vamedic)

The master of disaster looking to practice good medicine in austere environments


12.6: Types of Intermolecular Forces: Dispersion, Dipole–Dipole, Hydrogen Bonding, and Ion-Dipole. (2018, May 20). Retrieved August 5, 2019, from Libretexts website:

Woodring, J. H., & Reed, J. C. (1996). Types and mechanisms of pulmonary atelectasis. Journal of Thoracic Imaging, 11(2), 92–108.