Ask A Scientist Chemistry Archive

name         Ria B.
status       student
grade        9-12
location     CA

Question -   What is meant by hydrogen bonding?  I have studied a 
 little about Aqueous Solutions, and the term came up a few times.... but 
 sadly, I fail to know what it means.  If you could help me to understand, 
 I would greatly appreciate it.
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>ia,
>>e know that water has the chemical formula H2O, and that the way they are 
>onnected is H-O-H (that is, with the oxygen in the middle). Now, imagine 
> container wherein we have a bunch of water molecules, a bunch of H-O-H. 
>hen, imagine a second container wherein we have another bunch of water 
>olecules, only this time, the hydrogens in these particular water 
>olecules have been marked - maybe something like this: H*-O-H* (never 
>mind how we can do this). Now suppose we poured all of the water in 
container 2 into container 1. After a few seconds, we inspect the mixed 
water molecules and what we find is that a large percentage of the water 
has become H-O-H* (or H*-O-H). What can you conclude from this? Think 
about it first before reading the next paragraph.

We must conclude that somehow, water exchanges hydrogens very easily and 
very rapidly.

Now, we combine this idea with other information such as other molecules 
which are about the same as water in mass, and size (and other chemical 
properties) but we know do not exchange hydrogens the way water does. We 
find that these other molecules tend to be gases at room-temperature - 
which means that these molecules do not attract each other with the same 
strength as water - which is a liquid at room-temperature. We therefore 
have to conclude that this rapid exchange of hydrogens that water exhibits 
must strengthen its attraction with other water molecules.

This then is hydrogen bonding, an explanation for why water molecules 
exhibit a much stronger intermolecular attractive force with other water 
molecules, and which comes from the fact that the hydrogen-exchange causes 
a "bridging" effect between water molecules and holds them tightly to each 
other - tighter than other molecules of the same size would.

Greg (Roberto Gregorius)
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The element hydrogen forms a special type of bond unique to it -- hence
the name. Because hydrogen has only a single electron, when it forms a
bond with another element that is electronegative (that is, an element
that has a strong attraction for electrons such as F, Cl, O on the
right hand side of the periodic table) the positive charge of the proton of
the hydrogen is left partially "exposed" since there are no "inner"
electrons to shield the proton from surrounding electrons, and the single
electron from hydrogen is preferentially pulled toward the
electron-attracting element. This partial positive charge can then form
weak bonds by attraction from electrons in other molecules. While in
absolute terms of "bond strength" these hydrogen bonds is weak (about 1-3
kcal/mol compared to 10-30 kcal/mol for "regular" chemical bonds) they are
strong enough to have a large effect on the properties of hydrogen-bonded
molecules. In fact it is hydrogen bonds that hold the strands of  DNA in
the double helix, and account for the unusual properties of water, such as
its high boiling point and its large solvency for ionic and polar solutes.

Vince Calder
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Hi Ria!
In order to understand  what is "hydrogen bonding"
you must go a little before  at bonds in general.
I hope you know that covalent bonds and electrovalent
bonds strictly exist only  in extreme cases. Actually
when  atoms (and ions) relate to form some kind of
combination  both types happens (in different
proportions, of course)  in order that  the minimum
energy state is achieved. Then the concept of bond
polarity  is applied  as justified by  the molecular
bond theory.
Now the so called "polar bond"  (mostly covalent
bond)  have partial ionic  character.
Ions interact with molecules about the same way
they interact with ions of opposite charge.

If the molecule is polar it is oriented in the ion
electric
field  so that the dipole  end of the opposite charge
is directed toward the ion.  In any case, molecule is
polar or not, the ion presence  forms a dipole moment.
in the molecule.
There are different ways that can happen . One is
called  "permanent dipole-permanent dipole" and one of
them is what you  want to know: the hydrogen bond.
Usually hydrogen has a valence number of  1- (minus 1)
Then it can  share an electron pair forming a covalent
bond or can gain an electron an form an hidride ion.
When  bonded covalently to a highly  electronegative
element(F, N, O)  the hydrogen atom  carries some
high partial charge and due to its  very low positive
nucleus (1 proton) it is possible for it to get close
to a second  ion with partial negative charge.
Then there  happen a formation of  a strong
dipole-dipole bond.
  In the water case it is represented as:
    O--------H...............................O
So  one  hydrogen  in the water system  has a strong
covalent bond to one oxygen but is also attracted
to another oxygen from another water molecule.
I hope  that helps you
Thanks for asking  NEWTON!

Mabel
(Dr. Mabel Rodrigues)
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