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Interactions between two or more molecules are called intermolecular interactions, while the interactions between the atoms within a molecule are called intramolecular interactions. Intermolecular interactions occur between all types of molecules or ions in all states of matter. They range from the strong, long-distance electrical attractions and repulsions between ions to the relatively weak dispersion forces which have not yet been completely explained. The various types of interactions are classified as (in order of decreasing strength of the interactions):
Without these interactions, the condensed forms of matter (liquids and solids) would not exist except at extremely low temperatures. We will explore these various forces and interactions in the gas phase to understand why some materials vaporize at very low temperatures, and others persist as solids or liquids to extremely high temperatures.
Ion - Ion Interactions
Ion - Ion Interactions in the
The combination of atoms or ions is no longer a pair of
ions, but rather a polar molecule which
has a measureable dipole moment.
The dipole moment (D) is defined as
if there were a positive (+q) and a
negative (-q) charge separated by a
D = qr
If there is no difference in electronegativity between the atoms (as in a diatomic molecule such as O2 or F2) there is no difference in charge and no dipole moment. The bond is called a covalent bond, the molecule has no dipole moment, and the molecule is said to be non-polar. Bonds between different atoms have different degrees of ionicity depending on the difference in the electronegativities of the atoms. The degree of ionicity may range from zero (for a covalent bond between two atoms with the same electronegativity) to one (for an ionic bond in which one atom has the full charge of an electron and the other atom has the opposite charge). In some cases, two or more partially ionic bonds arranged symmetrically around a central atom may mutually cancel each other's polarity, resulting in a non-polar molecule. An example of this is seen in the carbon tetrachloride (CCl4) molecule. There is a substantial difference between the electronegativities of carbon (2.55) and chlorine (3.16), but the four chlorine atoms are arranged symmetrically about the carbon atom in a tetrahedral configuration, and the molecule has zero dipole moment. Saturated hydrocarbons (CnHn+2) are non-polar molecules because of the small difference in the electronegativities of carbon and hydrogen plus the near symmetry about each carbon atom.
Polar molecules can interact with ions:
Ion - Dipole
or with other polar molecules:
- Dipole Interactions
The charges on ions and the charge separation in polar molecules explain the fairly strong interactions between them, with very strong ion - ion interactions, weaker ion - dipole interactions, and considerably weaker dipole - dipole interactions. Even in a non-polar molecule, however, the valence electrons are moving around and there will occasionally be instances when more are on one side of the molecule than on the other. This gives rise to fluctuating or instantaneous dipoles:
Fluctuating Dipole in a Non-polar
These instantaneous dipoles may be induced and stabilized as an ion or a polar molecule approaches the non-polar molecule.
Ion - Induced Dipole Interaction
- Induced Dipole Interaction
It is possible that these forces arise from the fluctuating dipole of one molecule inducing an opposing dipole in the other molecule, giving an electrical attraction. It is also possible that these interactions are due to some sharing of electrons between the molecules in "intermolecular orbitals", similar to the "molecular orbitals" in which electrons from two atoms are shared to form a chemical bond. These dispersion forces are assumed to exist between all molecules and/or ions when they are sufficiently close to each other. The stronger farther-reaching electrical forces from ions and dipoles are considered to operate in addition to these forces.
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