Intermolecular Forces:  "Minor League Bonds"

I.  "Minor League Bonds" are called intermolecular forces.

   A.  Intermolecular "bonds" are forces of attraction among molecules.

   B.  The type of intermolecular bond shown among molecules depends on the polarity of the molecule.

   C.  The word electronegativity is often used in describing intermolecular forces:

            1.  Electronegativity is the attraction that an atom has for electrons in a bond.

            2.  Atoms at the upper right of the periodic table have high electronegativities

                    a.  Atoms with high electronegativities are "electron hogs."

                     b.  Fluorine, chlorine, and oxygen have high electronegativities.

            3.  Atoms at the lower left of the periodic table have low electronegativities.

                    a.  Atoms with low electronegativities have a low attraction for electrons.

                    b.  Francium and cesium have low electronegativities.

            4.  Since noble gases do not bond, they have no electronegativity value.

                    a.  Noble gases are already surrounded by an octet of electrons.

                    b.  Noble gases have NO attraction for extra electrons.

II.  Overview: Names of Intermolecular Forces

        1.  Polar molecules exhibit the following types of "minor league" intermolecular forces:

            a.  Dipole-Dipole Attraction:  an attraction among all polar molecules   

                    i. A dipole is a molecule that has two difference "ends" or poles.   CH₃Cl  is an example of a dipole.

                    ii. The end that is partially negative has an atom with high electronegativity.

                    iii.  The end that is partially positive has atom(s) with lower electronegativity.

            b.  Hydrogen Bonding:  the strongest dipole-dipole attractions in very small, polar molecules

                    i.  Water, H₂O

                    ii.  Ammonia, NH₃

                    iii.  Hydrogen fluoride, HF

        2.  Nonpolar molecules exhibit the following type of "minor league" forces:

            a.  Dispersion Forces

            b.  Nonpolar molecules like hexane, C₆H₁₄ exhibit dispersion forces_.

III.  Attractions Among Polar Molecules

   A.  Dipole-Dipole Attractions: 

        1.   A dipole attraction is the attraction that polar molecules have for other polar molecules.

        2.  The partial negative end of one polar molecule attracts the partial positive end of another molecule.

        3.  The smaller the polar molecule, the stronger the dipole attraction among them.    

        4.  Remember, "dipole" is a synonym for "polar molecule."

   B.  Hydrogen Bonding

        1.  Hydrogen bonds are a type of dipole attraction.  

        2.  Hydrogen bonding is an especially strong type of dipole attraction.

        3.  Hydrogen bonding only occurs in water (H₂0), ammonia (NH₃), and hydrogen fluoride (HF).

                a.  These are very small, extremely polar molecules.

                b.  The picture below illustrates the hydrogen bond (intermolecular force) in water:

Important:  Note that the partial positive end of water (hydrogen) is attracted to the partial negative end (oxygen) of the neighboring water molecule.  The intermolecular attraction is noted by the dashed lines.

    4.  Remember that hydrogen "bonding" is an intermolecular "minor league" attraction.

            a.  A hydrogen bond is never stronger than an ionic bond or a covalent bond!

IV.  Attractions Among Nonpolar Molecules

    A.  Dispersion forces are the intermolecular forces among nonpolar molecules.

            1.  They are caused by the random motion of electrons in nonpolar molecules.

            2.  A temporary imbalance of electrons creates a momentary dipole: 

            3.  Too many electrons on one side of the molecule creates a partial negative charge.

            4.  Too few electrons on one side of the molecule creates a partial positive charge.

    B.  Dispersion forces are attractions between the momentary dipoles of molecules.

            1.  Small nonpolar molecules have small dispersion forces.

                    a.  Small molecules have fewer electrons to imbalance.

                    b.  Small nonpolar molecules are often gases at room temperature: weak intermolecular forces.

                                i.  fluorine, F₂

                                ii.  carbon dioxide, CO₂

            2.  Large nonpolar molecules have larger dispersion forces.

                    a.  Large molecules have more electrons that may become "unbalanced."

                    b.  Larger nonpolar molecules are often liquids or solids at room temperature.

                                i.  bromine, Br₂ (liquid:  stronger intermolecular forces)

                                ii. iodine, I₂ (solid:  even stronger dispersion forces!)

            3.  Remember, a dispersion force is never stronger than an ionic or covalent bond!