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Hydrogen Bonds

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

A hydrogen bond is an extremely strong bond between molecules with a Hydrogen atom bonded to a Fluorine, Oxygen or Nitrogen atom and a molecule with a Fluorine, Oxygen or Nitrogen atom. Hydrogen bonds are notably found between the bases in DNA.

Well you may have read in textbooks this kind of definition for hydrogen bonds that the weak attractions between partially positive hydrogens and partially negative oxygens or nitrogens. You may not really get that cause you know what ionic and covalent bonds are but how does that fit into this? Hydrogen bonds are a weird category of bonds all kind of by themselves because unlike ionic or covalent bonds which hold individual atoms together to form molecules, hydrogen bonds are typically weak attractions between two different molecules that hold those two different molecules close to each other. Now how does this work? Well, it's all based on this quality that atoms have called electro negativity. In short electro negativity is essentially how strongly does that atom pull on its electrons.

Now carbon here I see a methane molecule CH4. It has roughly the same electro negativity as the hydrogens that are bound to it, indicated by these black lines because as roughly the same electro negativity. The two electrons here that form this bond are shared equally between the carbon and the hydrogen, same over here. Oxygen on the other hand is electro negative strong one. So it tends to pull the electrons much closer to it and further away from the hydrogen which is too weak to hold on to these electrons very well. So these electrons spend most of their time orbiting around the oxygen, very little time orbiting around the hydrogen. They are essentially having only visitational rights on the weekend as opposed to equal co-rights. So what that means is that these negative electrons tend to spend a little bit more time around the oxygen than around the hydrogen. That makes this oxygen slightly negative. If it's slightly negative that means this hydrogen here is slightly positive. Now scientists don't like writing stuff that means it's easy to understand. So they use the Greek letter delta [IB] delta to represent this slightly. I've always thought of it as kind of this weirdly twisted S, so I just think sort of. So here's my partially positive hydrogen or sort of or slightly positive hydrogen. So slightly positive, slightly positive, wait a second this is slightly negative but I know that water is a neutral atom, or a neutral molecule, so it's got to be, yes you guessed it, two slightly negatives, alright?

Now how this comes into play is here's another water molecule, this sort of positive, this is sort of positive this is two sort of negatives, this is sort of positive, this is sort of positive, this is two whoop slightly negatives. So, what happens is that as this water molecule is floating around, this water molecule over here, this is slightly positive, slightly negative and slightly attracted. You form this weak attraction between these two molecules. Similarly, this water molecule over here, its hydrogen is going, hey, "you're sort of attractive" and so we have our weak hydrogen bond there.

This is extremely important in many ways cause you, you may think of this as big whooptidu but it's actually a very big whooptidu in Biology. Because this influences tons of things especially water. Now you may have studied water in Chemistry and learned about all of its wonderful properties most of which are due to hydrogen bonding. For example, water is extremely hard to get it to boil. It takes a ton of energy. Why is that? Why does it take so much energy to turn it into a gas? That's because all the other water molecules start trying to cohere trying to stick to the water molecule that's going away pulling it back kind of like molecular Velcro. It's really hard to get it to go away. It takes a ton of energy. The same quality of water, this cohesiveness is how tree is able to evaporate water from its leaves and by cohesion it's able to pull water molecules all the way down from the bottom of the tree in the roots, pulling the water in from the soil.

Another ability that water has is adhesion, it tends to stick to things. Here is a standard paper towel. I put it on my hand and it starts to fall, no big deal. Why? Gravity. If I get it wet on the other hand, add some water. Now, I will show you some magic don don don. So same paper towel, all I've done is add water which should make it heavier so it should fall faster, right? And this [IB] was right, wait a second, it's not falling. Well eventually does, why? Did I generate antigravity here? Why did it stay? Because of hydrogen bonds. The water molecules were binding and sticking to the molecules that make up the paper towel and also binding and sticking to my hand, so acting as a molecular glue. Now, you may be thinking, okay so we can induce some evaporation stuff. So it's just water, no it's more, proteins, hydrogen bond all over the place and this gives them many of their abilities to form different shapes. It even causes the double helix of the DNA to spiral up and hold together. It is involving so many things that this is a trick that I've taught my AP Biology students. Whenever I call them and they don't know the answer and the answer that they can always pull out of their pocket is, "it's due to hydrogen bonds" and chances are it's right. If your teacher's one of those really annoying teachers, if he knows lots of stuff and he's really smart knows a lot and he calls on you, just say well, it's probably due to hydrogen bonds and if he's smart enough, he'll be able to invent a way that yes it is due to hydrogen bonds.