What is a hydrogen bond?

Question

What is a hydrogen bond? How does polarity help hydrogen "bonds" form?  And how is intermolecular force different from intramolecular force? 

Answer 1

Hi Sara.

 

Hydrogen bonds are weak intermolecular attractions that form usually when there's a hydroxyl group or an amine group.

 

-OH, hydroxyl group

 

-NH₂ , amine group

 

Because O and N are highly electronegative, it will cause the electron in the bond between them and hydrogen to be closer to them than the electron is to hydrogen. This gives hydrogen a partially positive charge. Since the electron is so near to O and/ or N, it will give them a partially negative charge.

 

Take water for example.

 

H - O - H

 

Because of the strong electronegativity of the O in comparison with the H's, both H's will be partially positive, and the oxygen will be partially negative.

 

+   -    +

H - O - H

 

The partial positive charge on the hydrogen of one hydroxyl/ amine will go and find the partial positive charge on another hydroxyl/ amine. It won't form an actual bond, it'll just be strongly attracted to it.

 

         H

        /

      O

      |

      H

H - O - H

          O

        /   \

      H     H

 

The polarity of the OH bond will allow the hydrogen bonds to form between the water molecules. Hydrogen bonds, though weak, are incredibly important in the stabilization of proteins and DNA. This extensive hydrogen bonding in water is also why it takes so much energy to boil it.

 

Inter-molecular forces are the ones like hydrogen bonds; they happen from molecule to molecule.

 

Intra-molecular forces are the forces that hold a molecule together.

 

Hope this helps.