Intermolecular Forces
Definition and meaning of Intermolecular Forces in chemistry.
Intermolecular forces are the physical attractive forces that act between separate neighboring molecules. They also act between individual unbonded atoms in noble gases. These forces are distinct from the strong intramolecular forces that hold atoms together within a single molecule.
In more detail
There are three main types of attractive intermolecular forces found in neutral chemical substances. London dispersion forces are temporary induced-dipole attractions that are present in absolutely all molecules. Dipole-dipole forces exist only between specific molecules that possess permanent electrical dipoles.
Hydrogen bonding is an especially strong and highly specific type of dipole-dipole interaction. It occurs specifically when a hydrogen atom is directly bonded to nitrogen, oxygen, or fluorine. These intermolecular forces are typically much weaker than true internal covalent or ionic bonds.
However, they completely govern the bulk physical properties of all macroscopic molecular substances. They directly determine physical properties like boiling point, melting point, fluid viscosity, and solubility. Stronger intermolecular forces generally lead to higher boiling and melting points for a given substance.
This happens because more thermal energy is physically required to separate the strongly attracted molecules. A common student mistake is believing that boiling a liquid breaks its internal covalent bonds. Boiling simply overcomes the weaker intermolecular forces while leaving the individual molecular structures completely intact.
Key facts
| Field | General Chemistry |
|---|---|
| Main types | London dispersion, dipole-dipole, hydrogen bonding |
| Typical strength | 1 to 40 kJ/mol |
| Comparison | Much weaker than covalent or ionic bonds |
| Key effect | Determines boiling point, melting point, and viscosity |
| Weakest force | London dispersion forces |
Water (H2O) boils at 100 degrees Celsius under standard atmospheric pressure conditions. Hydrogen sulfide (H2S) is a structurally similar molecule from the exact same periodic group. Despite being noticeably heavier, hydrogen sulfide boils at a freezing negative 60 degrees Celsius. Water molecules can form very strong hydrogen bonds with each other in the liquid state. Hydrogen sulfide molecules only experience much weaker dipole-dipole interactions and London dispersion forces. The uniquely strong intermolecular forces in water directly cause its drastically higher boiling point.
Frequently asked questions
How do intermolecular forces differ from intramolecular forces?
Intramolecular forces are the strong chemical bonds that hold atoms together within a single molecule. Intermolecular forces act between separate distinct molecules and are significantly weaker.
Which type of intermolecular force is considered the strongest?
Among the forces common to neutral molecules, hydrogen bonding is generally the strongest. Dipole-dipole forces are moderately strong, and London dispersion forces are typically the weakest.
Do all chemical molecules experience London dispersion forces?
Yes, all molecules experience London dispersion forces because they all contain constantly moving electrons. These moving electrons create temporary uneven charge distributions that attract neighboring molecules.