J.R. S. answered • 08/18/17
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Ph.D. in Biochemistry--University Professor--Chemistry Tutor
First, let us define "hydrogen bond". It isn't really a bond, per se, but a strong attraction. In order for a hydrogen bond to exist, a H atom must be BONDED to either a N atom, O atom, or F atom, as for example, in H2O, NH3, or HF. Note: these are actual covalent bonds where H (hydrogen) is part of a molecule. Now, when you have this case, you have a large difference in electronegativity between the H and the N, O, or F, such that these more electronegative atoms "pull" electrons toward them and away from H. This creates a dipole, where the H is partially + and the N, O or F is partially -. Taking HF (hydrogen fluoride) as an example, the partially positive H will then be attracted to the partially negative F of another HF molecule, and so on and so on, and this is the so-called hydrogen bond (not really a bond, but an attraction). In diagram form, it might look like this:
H-F --- H-F where the dotted line is the hydrogen bond between a partially negative F and a partially positive H between two molecules of HF.
Does this make sense? If not, message me, or comment.
J.R. S.
tutor
1. H hasn't "given" it's electrons to N, O or F, but rather has shared its electrons. Since these other atoms are considerably more electronegative than H, the shared electrons spend more time around the N, O or F than around the H. This is what I meant by "pulled away".
2. This is because of the electronegativity of H and the properties that make it a non metal. To keep it simple, H has a fairly low electronegativity value (~2) and N, O and F have high values (3 - 4). There are other atoms that have lower EN values than H, but they don't enter the discussion because they are metals, and when a metal and non metal bond, it is more ionic than covalent. This may be beyond the scope of this discussion, however. This is why only H is discussed in H bonding.
3. See #2 above. Even though H is in Group I, it is NOT considered to me a metal, like Li, Na, etc. It obviously has different properties. For one thing, it is the only element that does not contain a neutron.
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08/18/17
Justin J.
Yeah I could understand right now. Could you tell whether hydrogen can exist without any electrons if it already have shared it's electrons with N or O. Is it because hydrogen H+ can't exist it attracts to another molecule? Is hydrogen the only element that makes these type of attractions to another molecule.
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08/18/17
J.R. S.
tutor
Yes, of course hydrogen can exist without any electrons. It is known as a proton, H+. This is what makes an Arrhenius acid and a Bronsted acid an acid. They donate protons. If you look at HCl (a strong acid), when place in water, it dissociates to H+ and Cl-.
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08/18/17
Justin J.
Okay. Is there any special property for Hydrogen bond? I can't get why the name of hydrogen element has been given to an attractive force. Could you tell why doesn't IUPAC doesn't use any other element's name for attractive forces made by other elements excluding hydrogen?
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08/18/17
J.R. S.
tutor
The term "hydrogen" bond comes about because it only occurs when H is involved in the intermolecular force. And, BTW, that's all that a hydrogen bond is. It is one type of intermolecular force. Some others are dipole-dipole, induced dipole, dispersion force, etc. Why the term hydrogen bond developed, per se, I cannot tell you, but it is the strongest of all the intermolecular forces. If you look at the periodic table and consider any other non metallic element bound to N, O or F, you can see that the ?EN is not as great as that for when H is bound to these elements. This is what makes H unique and what makes hydrogen bonding unique.
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08/19/17
Justin J.
Yeah that's great.
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08/20/17
Justin J.
08/18/17