Attracting other molecules: Intermolecular forces

A polar molecule is a dipole — with one end having a partial negative charge and the other end having a partial positive charge — so it acts like a magnet. These charged ends can attract other molecules. For instance, the partially negatively charged oxygen atom of one water molecule can attract the partially positively charged hydrogen atom of another water molecule. This attraction between the molecules occurs frequently and is a type of intermolecular force (force between different molecules).

Intermolecular forces can be of three different types:

✓ London force (dispersion force): This very weak type of attraction generally occurs between nonpolar covalent molecules, such as nitrogen (N₂), hydrogen (H₂), or methane (CH₄). It results from the ebb and flow of the electron orbitals, giving a very weak and very brief charge separation around the bond.

✓ Dipole-dipole interaction: This intermolecular force occurs when the positive end of one dipole molecule is attracted to the negative end of another dipole molecule.  It’s much stronger than a London force, but it’s still pretty weak.

✓ Hydrogen bond: The third type of interaction is really just an extremely strong dipole-dipole interaction that occurs when a hydrogen atom is bonded to one of three extremely electronegative elements: O, N, or F. These three elements have a very strong attraction for the bonding pair of electrons, so the atoms involved in the bond take on a large amount of partial charge. This bond turns out to be highly polar — and the higher the polarity, the more effective the bond.

  When the O, N, or F on one molecule attracts the hydrogen of another molecule, the dipole-dipole interaction is very strong. This strong interaction (only about 5 percent of the strength of an ordinary covalent bond but still very strong for an intermolecular force) is called a hydrogen bond. The hydrogen bond is the type of interaction that’s present in water.