Hydrogen bonding can be so confusing, so I
want to talk about some common mistakes people make so you won't ever make them. Okay, so hydrogen bonds are a type of intermolecular
bond. Intermolecular bonds form between molecules,
that's why we call them intermolecular. Now this "between" part is super important. Between two different molecules, I'm underlining
it twice in red. Okay, let's look at hydrogen bonds in some
ethanol molecules. So, ethanol's an alcohol and if we could take
a drop of ethanol and zoom in zillions and zillions of times in it this is what we'd
see. We'd see a bunch of different molecules all
twisting around to different orientations and here they are. So, where are the hydrogen bonds in these
molecules? Well as I said earlier, intermolecular bonds,
hydrogen bonds, form between molecules, okay? So this can't be a hydrogen bond because it's
not between molecules, okay? The CH, that can't be a hydrogen bond either
because it's in the same molecule. It's not between molecules. In the CC, it can't be a hydrogen bond either
because it's in the same molecule, not between different molecules. So that's the first important thing to keep
in mind; if a bond is in just one molecule it's not an intermolecular bond and it's certainly
not a hydrogen bond. Okay, so that means that the hydrogen bond
has to be between different molecules. Here's how it works. In hydrogen bonding, in this case, an H connected
to an O is bonded to an O on another molecule. Okay, so this H is attached to an O and so
it will form a hydrogen bond with this O over here. I'll indicate this hydrogen bond with a couple
red dots so there is a hydrogen bond here. This H connected to the O here can form a
hydrogen bond with this O over here on this molecule. So again, a hydrogen bond I'll indicate with
a couple of these red dots here. So there's a hydrogen bond here. Okay now, a common mistake that some people
make is they think anytime there is an H, it can form a hydrogen bond between molecules. They say there's an H here connected to the
C, there's an H here connected to the C, so this can form a hydrogen bond. Right? No. It can't. It has to be an H connected to an O bonding
with another O in this case. So it can't be this H with the C and this
H with the C, that's not a hydrogen bond. Another mistake is taking a H that's connected
with the C and connecting it with an O. That's kind of half right because in this case O's
are forming hydrogen bonds but if the H is connected with a C it cannot form a hydrogen
bond either. It has to be an H connected with an O making
a bond with an O on another molecule. This is wrong, this is wrong, but these two
hydrogen bonds between the molecules is the way to go. Okay, so now that you know how to find and
draw hydrogen bonds, let's talk about how hydrogen bonds form in
the first place. Here I have two ethanol molecules; they're
going to form a hydrogen bond between them. In this case, it's all about the O and the
H. So the O and the H in this molecule are connected to each other with this line which
indicates that it's a covalent bond that's holding them together. They're sharing electrons to form this covalent
bond so they're stuck together because they're sharing electrons. Here's the thing though with the sharing,
they are not sharing those electrons equally, okay? The oxygen has what we call a higher electronegativity. All that means is that when it shares with
hydrogen, it hogs the electrons. So it's maybe supposed to be sharing equally
but instead it's hogging the electrons. Because it has all these extra electrons that
it's hogging, it has a little bit of a minus charge because of those electrons that are
spending more time with it. So we can write this minus sign here in this
squiggle that's actual a lower case Greek Delta letter D. We have this lower case Greek Delta with a
negative sign meaning the O is a little bit negative because it's hogging those electrons. On the other hand, hydrogen here is getting
the electrons hogged away from it. So it's missing those electrons so it has
lower case Delta plus, it's a little bit positive. Now the same thing is happening over on this
molecule. The oxygen is hogging an electron so it has
a Delta minus sign because it has a little bit of a negative charge; those extra electrons
that it's hogging away. An H that is getting thieved away from here,
it has a little bit of a positive charge. So now you can see what's going on here. The positive hydrogen here is going to be
attracted to the negative oxygen here. So this attraction, it's sort of like magnets. You know how magnets attract together? It's like this attractive force between the
positive hydrogen and the negative oxygen. So we can say that the hydrogen bonds formed
because O is hogging electrons from H. It's making O a little bit negative, it makes H
a little positive so this positive H is going to be attracted to a negative O on another
molecule. Now here's the thing, earlier I said that
if an H is bonded to a C it will not form a hydrogen bond. Here's why. It's because C and H share their electrons
evenly in this spot. Because they're sharing them evenly, it's
not possible for the C to become negative and the H to become positive because the electrons
spend equal time with both of them. So you don't get these negative and positive
charges when H is bonded to a C since there are no charges. H's bonded to C's can't form hydrogen bonds. So it turns out that O and H aren't the only
atoms that can form hydrogen bonds. We saw just a minute ago that O when it's
bonded to H, O hogs those electrons away from H. It's not the only atom that can do that. Fluorine also has a high electron negativity
which means it likes to hog so fluorine when it's bonded to an H also has a little bit
of a negative charge making the hydrogen get a little bit of a positive charge. Nitrogen(II) is able to hog electrons away
from hydrogen so it gets a negative charge getting hydrogen a positive charge. So there are three atoms that can participate
in hydrogen bonding besides hydrogen and they are F, O, and N. If a hydrogen is bonded to
a F, O, or an N that hydrogen can form a hydrogen bond. You can think about it because it spells FON,
not correctly, but F-O-N. So here's the thing, these guys all have to
attach, they have to be attracted to an atom on another molecule, okay? So for example, this positive H here can be
attracted to an O on another molecule because that O will be a little bit negative. But this O could also bind with an H attached
to an F. Or an O because it's a little bit negative could bond to an H if it's attached
to an N. Okay? So that's possible. Or you can have an F, a fluorine, bonding
with any of these H's if H is attached to it. F or an O or an N this slightly negative fluorine
will be able to hydrogen bond with a positive hydrogen. Similarly, nitrogen(II) when it's on another
molecule can form a hydrogen bond with any of these three combinations; the H to the
N, the H connected to the O, or the H connected to the F. So whenever you see a hydrogen connected
to a fluorine, oxygen, or nitrogen that means that it can form a hydrogen bond. Any of the hydrogens can form a hydrogen bond
with a nitrogen, fluorine, or oxygen on another molecule. So that is hydrogen bonding.