Intermolecular bonding

Dipoles are molecules that have a positive end and a negative end, due to the polar bonds. There are three main types of dipoles:

• Permanent dipoles occur when the two atoms in a covalent bond have very different electronegativities. The atom that is more electonegative is slightly negative and the other is slightly positive
• Instantaneous dipole
• Induced dipole

These can act between all molecules, including molecules with permanent dipoles because instantaneous dipoles can occur elsewhere in the molecules. Instantaneous-induced dipole bonds are more noticeable in non-polar substances. They are the weakest type of intermolecular bond. As the number of electrons the atoms have increases, the greater the instantaneous-induced dipole effect.

These work with a similar principle to instantaneous-induced dipole bonds, but one molecule has to have a permanent dipole. An example is between hydrogen chloride and chlorine molecules.

Permanent-permanent dipole bonds are relatively strong, compared to instantaneous-induced and permanent-induced bonds. Therefore, molecules with permanent-permanent dipole bonds are more likely to be liquids or solids.

Hydrogen bonds are the strongest type of intermolecular bonds, and can be thought of as special types of permanent-permanent dipole bonds. In order for hydrogen bonding to take place, the molecules involved have to have the following features:

• A large dipole between a hydrogen atom and a highly electronegative atom, like nitrogen, oxygen, or flourine
• A small hydrogen atom that can get very close to nitrogen, oxygen, or flourine atoms in nearby molecules
• A lone pair of electrons on the nitrogen, oxygen, or flourine atom