Intermolecular Forces
Definition and meaning of Intermolecular Forces in chemistry.
Intermolecular forces are the attractive forces that act between separate molecules (or between individual atoms, as in noble gases), rather than the forces holding atoms together within a molecule.
In more detail
They include London dispersion forces (temporary, induced-dipole attractions present in all molecules), dipole-dipole forces (between molecules with permanent dipoles), and hydrogen bonding (an especially strong dipole-dipole interaction when H is bonded to N, O, or F). These forces are typically 10 to 100 times weaker than covalent or ionic bonds, but they govern bulk physical properties such as boiling point, melting point, viscosity, and solubility. Stronger intermolecular forces generally mean higher boiling and melting points, since more energy is needed to separate the molecules.
Key facts
| Field | General Chemistry |
|---|---|
| Main types | London dispersion, dipole-dipole, hydrogen bonding, ion-dipole |
| Typical strength | ~1-40 kJ/mol (vs. ~150-1100 kJ/mol for covalent/ionic bonds) |
| Key effect | Determines boiling point, melting point, viscosity, and solubility |
Water (H2O) boils at 100°C while hydrogen sulfide (H2S), a heavier molecule from the same periodic group, boils at -60°C, because water molecules form hydrogen bonds with each other while H2S molecules cannot.
Frequently asked questions
How do intermolecular forces differ from intramolecular forces?
Intramolecular forces (covalent, ionic, or metallic bonds) hold atoms together within a molecule or compound. Intermolecular forces act between separate molecules and are much weaker, which is why substances melt and boil without breaking their covalent bonds.
Which intermolecular force is strongest?
Among the forces common to neutral molecules, hydrogen bonding is generally the strongest, followed by dipole-dipole forces, with London dispersion forces being the weakest, though dispersion forces can dominate in large molecules with many electrons.