Intermolecular forces are electrostatic forces between atoms and molecules, including ion-dipole, hydrogen bonding, dipole-dipole, and London dispersion forces.
These forces depend on factors such as electronegativity and dipole moments. Ion-dipole forces are present in ionic substances dissolved in polar solvents, while dipole-dipole forces occur between polar molecules. London dispersion forces occur due to the formation of instantaneous dipoles and are greater for larger molecules. Hydrogen bonds occur between hydrogen atoms and electronegative atoms and are responsible for the high boiling point of water. Intermolecular forces affect the physical characteristics of chemical species.
In chemistry, intermolecular forces describe various electrostatic forces present between atoms and molecules. These forces include ion-dipole forces, hydrogen bonding, dipole-dipole forces, and London dispersion forces. While these forces are generally much weaker than ionic or covalent bonds, they can still have a large influence on the physical characteristics of liquids, solids, or solutions.
All intermolecular forces are electrostatic in nature. This means that the mechanics of these forces depend on the interactions of charged species such as ions and electrons. Factors related to electrostatic forces, such as electronegativity, dipole moments, ionic charges, and electron pairs, can greatly influence the types of intermolecular forces between two given chemical species.
Ion-dipole forces are present between ions and partial charges at the ends of polar molecules. Polar molecules are dipoles and have a positive and a negative end. Positively charged ions are attracted to the negative end of a dipole and negatively charged ions are attracted to the positive end of a dipole. The strength of this type of intermolecular attraction increases with increasing ionic charge and increasing dipole moments. This particular type of strength is commonly found in ionic substances dissolved in polar solvents.
For neutral molecules and atoms, intermolecular forces that may be present include dipole-dipole forces, hydrogen bonds, and London dispersion forces. These forces constitute the van der Waals forces, named after Johannes van der Waals. In general, they are weaker than ion-dipole forces.
Dipole-dipole forces occur when the positive end of one polar molecule approaches the negative end of another polar molecule. The force itself depends on the proximity of the molecules. The farther away the molecules are, the weaker the dipole-dipole forces. The magnitude of the force can also increase with increasing polarity.
London dispersion forces can occur between both non-polar and polar chemical species. They are named in honor of their discoverer, Fritz London. The force itself occurs due to the formation of instantaneous dipoles; these can be explained by the movement of electrons in chemical species.
Instantaneous dipoles are created when electrons around one chemical species are attracted to the nucleus of another chemical species. In general, London dispersion forces are greater for larger molecules, because larger molecules have more electrons. For this reason, for example, large halogens and noble gases have higher boiling points than small halogens and noble gases.
Hydrogen bonds occur between hydrogen atoms in a polar bond and unshared electron pairs on small ions or electronegative atoms. This type of intermolecular force is often observed between hydrogen atoms and fluorine, oxygen or nitrogen. Hydrogen bonds are found in water and are responsible for the high boiling point of water.
Intermolecular forces can have a profound effect on the physical characteristics of a chemical species. Typically, high boiling points, melting points and viscosities are associated with high intermolecular forces. Although much weaker than covalent and ionic bonds, these intermolecular forces of attraction are still important in describing the behavior of chemical species.
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