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Intermolecular Forces

Intermolecular forces determine the physical properties of liquids and solids.  Without intermolecular forces, nothing would exist in the liquid or solid state, everything would be in a gaseous state.

 Dispersion Forces

At first glance it is difficult to see why there would be intermolecular forces of attraction between helium atoms, but we know that these forces do exist because helium can be liquefied at about 5 K.  To explain this result, we must realize that the electron charge density that results in helium being nonpolar is an average one.  On average, the electron density associated with helium's two 1s electrons is evenly distributed around the nucleus.  But at any given instant, the electron charge density may become nonsymmetrical.  The normally nonpolar atom becomes momentarily polar and an instantaneous dipole is formed.  This momentary dipole can induce a dipole in a neighboring helium atom.  The force of attraction between an instantaneous dipole and an induced dipole is known as a dispersion force (also called a London force). 

Polarizability

The measure of the ease with which electron charge density is distorted by an external electric field.  In large atoms, the valence electrons are more loosely bound, so they can shift toward another atom more readily than can the more tightly bound electrons in small atoms.  This means that large atoms and molecules are more polarizable than small ones.  Which also means that polarizability increases with atomic and molecular weight.  Another factor affecting polarizability is molecular shape. The electrons in elongated molecules are more easily displaced than those in more symmetrical molecules.

Dipole-Dipole Forces

A polar substance, due to its shape and differences in electronegativities between bonded atoms, has permanent dipoles.  Polar molecules tend to align themselves so that the positive end of one molecule  is next to the negative end of another molecule.  Since all molecules have dispersion forces, the total intermolecular forces in polar molecules will be greater than in nonpolar ones since because the polar molecules have both dipole-dipole and dispersion forces at work. 

Hydrogen Bonds

We have one more type of intermolecular force to consider, and that is hydrogen bonds.  A hydrogen bond is an intermolecular force resulting from a hydrogen atom covalently bonded to a nonmetal atom in one molecule being simultaneously attracted to a nonmetal atom in a second molecule.  In order for a hydrogen bond to form the nonmetal must be small and very electronegative.  So generally we find hydrogen bonding only with nitrogen, oxygen, and fluorine.  Remember if a hydrogen atom is to be involved in a hydrogen bond it must be on an oxygen, nitrogen, or fluorine atom.







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