#"Bond strength"# essentially measures the strength of the bonds between ATOMS within molecules (or between atoms in non-molecular structures). This is #"intramolecular force"# because we address the strength of a bonding interaction within a discrete molecule.

The #H-H# bond is strong, i.e. we can measure an INTRAMOLECULAR bond strength of #432*kJ*mol^-1#. This is exceptionally strong as bonds go. Nevertheless, dihydrogen gas has a boiling point of #-252.9# #""^

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C#. What"s going on?

Well, the boiling point reflect the strength of interaction BETWEEN molecules of dihydrogen, where only weak dispersion forces operate. On the other hand the bond strength measures the degree of INTRAMOLECULAR interaction, the bond between the hydrogen atoms in a discrete hydrogen molecule.

For another example of INTER versus INTRAMOLECULAR FORCE consider the bond strength of #H-O# in the water molecule. And this has been measured at #460*kJ*mol^-1#, again strong as bonds go. The normal boiling point of water at #100# #""^
C# is unusually high for such a small molecule. The high boiling point reflects the strength of the BONDS between molecules, an INTERMOLECULAR force, and it is high because of the propensity of water to hydrogen bond, which is a potent intermolecular force. And likewise for #HF#, the normal boiling point #-=19.5# #""^
C#, is also unusually high, and this again can be attributed to hydrogen bonding, i.e. #""^(delta+)H-F^(delta-)# dipoles which align INTERMOLECULARLY.

This distinction between intermolecular and intramolecular force has been recognized as a problem area of A level and 1st year university chemistry, and I urge you to read the appropriate chapter of your text that addresses the issue.

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David Drayer
Mar 11, 2017

intramolecular forces are the bonding of atoms that holds a molecules together intermolecular forces hold molecules together in a solid or liquid.


Using water as an example

Intramolecular forcesThe bonds between hydrogen and oxygen hold the water molecule together. The sharing of electron density creates a bond that holds the two atoms together.

Intermolecular forces So called Hydrogen bonding. The difference between the electro negativity between hydrogen and oxygen creates a positive charge on Hydrogen and a negative charge on Oxygfen. The negative charge attracts the Hydrogen on a neighboring molecule. This helps to hold the water molecules together.

Intramolecular forces hold atoms together to form molecules.

Intermolecular forces hold molecules together to form solids and liquids.

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