By Robert Hazen, Ph.D., George Mason University
All of the complex molecules described in the world of chemistry are compact. They’re electrically neutral objects. That is, they’re collections of atoms that don’t need to give away any electrons, and they don’t have an electric charge. So, you might ask, ‘What kind of attraction could there be between neutral molecules?‘
How the Van Der Waals Interaction Works
Molecules are surrounded by electrons, so when two molecules approach each other, every electron, every proton in one molecule senses the presence of every electron and proton in the other molecule, and you start getting distortions of the electrons and protons. Just slight shifts, but enough that you get slight areas of positive charge and slight areas of negative charge.
And, it turns out that these can actually cause an attraction between molecules. That’s called the Van Der Waals interaction or Van Der Waals attraction, by which molecules can then bind together. This is a very common kind of bonding in minerals and almost any kind of layer minerals such as talc, which forms baby powder, talcum powder; also graphite in your lead pencil.
These are compounds in which Van Der Waals interactions hold layers of the material that are inherently electrically neutral together. Also, in many organic compounds, in candles, the waxy feel of the candle, or soap, or of many other organic compounds, is due to Van Der Waals interactions that are holding these molecules loosely together. They’re not ionically bonded or covalent bonded, or metallic bonded. It’s a separate kind of attraction.
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Polymers and Their Relationship with Hydrogen Molecular Bonding
Now the other kind of intermolecular force is called the hydrogen bond. There is a class of compounds that relies very heavily on hydrogen bonds. Most of the organic molecules are relatively small, with just a few dozen atoms. But there’s a very large class of organic molecules that has vast numbers, millions or billions or trillions of atoms. These are called polymers.
Now, a polymer is in widespread use in everyday life in plastics and many other biological materials: your skin, the fibers of your tendons, your hair. These are long-chain molecules that form the interaction of what begins as very small molecules. But they latch on one to another to make these incredibly long chains, chainlike molecules that then form most of the fibers in the natural world.
Well, plastics form then from complexly intertwined strands of polymers. The plastics you use in your everyday life are really interesting structures because it’s like a set of fibers, bundles of fibers interwoven. And yet, those fibers would normally just slide past each other unless there was some other kind of bonding, and that’s where hydrogen bonding comes in.
Learn more about isotopes and radioactivity.
How Hydrogen Bonding Works
If you think about the surface of a molecule, you’re going to have a certain number of hydrogen atoms just exposed on the surface. In other places, you may have oxygen atoms exposed at the surface. And these particular atoms tend to develop local charges. Hydrogens very often tend to be slightly positive on their surfaces, while oxygens very often tend to be very negative on the surface of an otherwise neutral molecule.
And so, if you can juxtapose two strands or two molecules so that the positive and negative charges become opposite to each other, you can then start forming very strong bonds between them. This is exactly what happens in hydrogen bonding. This is what makes plastics strong, even though they’re made of neutral molecules that are interacting.
Learn more about nuclear fission and fusion reactions.
Hydrogen Bonding: What Makes Ice Extremely Strong
It’s also an extremely important bond in everyday water. Take a look at the H2O molecules of water and think about ice. H2O is a neutral molecule. Two hydrogens, one oxygen in a neutral unit. And yet ice is an extremely strong substance. The reason is that every water molecule on ice is hydrogen-bonded to others. The hydrogens see other oxygen, the oxygens see other hydrogens, and this forms an intense cross-linking of these hydrogen bonds.
Let’s demonstrate that with a piece of ice. What you’re going to see in this demonstration is that ice actually behaves very much like an ionic compound. If you take a piece of ice, it’s transparent, just like ionic compounds. If you hit it with a hammer, it shatters, just like ionic compounds. It’s brittle, and indeed the pieces could be stuck right back together if you will.
A simple demonstration shows the similarity between the hydrogen-bonded ice and ionic bonding. Ice is transparent, and it’s hard. And if you break it with a hammer, it splits very much like ceramic. The resulting pieces then are sharp, they have edges, and they fit right back together, just like a ceramic cup when it’s broken.
Common Questions about Van Der Waals Attraction and Hydrogen Bonding
Van Der Waals attraction is a type of molecular bonding that causes a weak bond between molecules. Each molecule has a number of electrons and protons around it. When the electrons and protons of one molecule approach the electrons and protons of another molecule, they attract each other, and a Van Der Waals attraction is formed between them.
A hydrogen bond is a very strong molecular bonding that occurs between neutral molecules that interact. In this bonding, the negative hydrogen atoms of a molecule are bonded to the positive oxygen atoms of the adjacent molecule. For example, ice forms when hydrogen bonding occurs between water molecules.
Hydrogen bonding is a type of molecular bonding very similar to ionic bonding. Materials that are formed with these two bondings are transparent and very strong. These materials are brittle and break into smaller pieces when broken. In both hydrogen and ionic bondings, the broken parts can be reattached.
