Intermolecular Forces

 Intermolecular Forces – forces of attraction between molecules – act only between neighboring molecules  Boiling Point – good measure of the strength of the forces holding the particles together ◦ Higher boiling point = Greater intermolecular forces  Melting Point – also relates to the attractive forces between particles

 Ionic Compounds tend to be solids with high melting points (especially small ions)  Covalent Compounds tend to be gases and liquids and have low melting points  Covalent Compounds tend to melt at lower temperatures than ionic compounds

 Due to opposite charges, ionic substances have higher forces of attraction  This force is greater in smaller ions due to the shorter distance between the atoms  Smaller ionic compounds have high melting and boiling points  Compare NaCl and KCl. Which will have the higher melting and boiling points?  What about CaF 2 ?

 Interactions between polar molecules ◦ Polarity – refers to the degree of unequal sharing of electrons in a covalent bond ◦ Greater difference in electronegativity means greater polarity  Positive end of one molecule attracts the negative end of the neighboring molecule  Greater polarity means a stronger dipole- dipole force  Stronger dipole-dipole force means higher melting and boiling points

 Special dipole-dipole forces  Occurs when a hydrogen atom that is bonded to a highly electronegative atom is attracted to unshared electrons of another molecule  As electronegativity difference increases, boiling points increase (p. 387)

 Form between highly electronegative atoms in the upper right corner of the periodic table.  Strong dipole-dipole forces because ◦ Electronegativity differences ◦ Small size of the hydrogen atom ◦ Hydrogen has only one electron – when pulled away by a highly electronegative atom, the positively charged proton is partially exposed ◦ This exposed proton is strongly attracted to an unbonded electron pair of another molecule

 Hydrogen bonds account for many of water’s properties  Because two hydrogen atoms bond to one oxygen atom covalently, the hydrogen atoms can also bond to neighboring molecules.  Consequently, each hydrogen atom can form multiple hydrogen bonds  Intermolecular forces in water are very strong.  Solid water (ice) is less dense than liquid due to the structure of the water molecule.

 Intermolecular attraction resulting from the uneven distribution of electrons, and the creation of temporary dipoles  Form between nonpolar molecules so there is no “positive” or “negative” end to the molecules  Because the number of electrons increase as molar mass increases, London Disp. Forces also increase.  Remember that electrons are constantly in motion….consequently, temporary dipoles form as the electrons move around the molecules.

 Nonpolar molecules can ONLY experience London dispersion forces.  Ionic and Polar molecules can experience both dipole-dipole forces and London dispersion forces (although to a relatively small degree).  Increasing boiling points result from increasing strength of bonds. Ionic bonds are the strongest, followed by polar, and then nonpolar bonds.

 If molecules are very large, and/or nonsymmetrical, the effects of the intermolecular forces are diminished.