COMPOUNDS

Compounds are labeled as either ionic or covalent, depending on the types of bond that are holding them together. When you know the type of bonding that exists in the compound, then you can further classify the compound as either:

    1.    Ionic (meaning that it has ionic bonds and is composed of ions, or

    2.    Molecular (meaning that it has covalent bonds and is composed of molecules.)

To further describe these:

    1.     An Ionic compound is a compound which consists, not of atoms, but of positive and negative ions.

            Usually ionic compounds form between metals and nonmetals, but you need to check for the

            electronegativity difference (I.7 or greater) before labeling a compound as ionic. An example

            of an Ionic Compound is NaCl. Its structure may be shown as Na⁺Cl^-.

            In Ionic Compounds the attraction between the (+) and (-) charges is very strong, not only within

            a given formula unit (Na⁺ to Cl^-), but also between units (Na⁺ to Cl^- to Na⁺ to Cl^-, etc.) In fact, the

            ions are held together in a crystal lattice, and Ionic Compounds have predictable properties; they

            are hard, brittle, crystalline solids with high melting points.

    2.    A Molecular compound, on the other hand, consists of molecules, and molecules are made up

           of atoms that are sharing electrons in order to be satisfied. Usually molecular compounds form

           between 2 nonmetals, but, again, you need to check for the electronegativity difference (less

           than l.7) to be certain.

           Question:        Which of the following compounds are molecular?

                                    (a) H₂O    (b) KBr    (c) H₂    (d) P₂O₅    (e) N₂O

        Answer:         (a), (c), (d), and (e) all consist of nonmetals, so you would expect that these

                            compounds would be molecular. A check of the electronegativity table reveals

                            the following electronegativity differences:

                            (a)    Molecular (electronegativity difference - 1.4 = covalent bonding);

                            (b)    Ionic (difference = 2.0 = ionic);

                            (c)    Molecular (electronegativity difference = 0.0)

                            (d)    Molecular (difference = 1.4 = covalent);

                            (e)    Molecular (difference = 0.50 = covalent).      

Forces Between Molecules:

Remember that Molecular Compounds consist of atoms held together by covalent bonds. The forces that hold   molecules together within a molecular compound are known as Intermolecular Forces. These Intermolecular Forces (forces between molecules) can be classified as:

        1.    Dipole to Dipole Forces;

        2.    London Forces; and

        3.    Hydrogen Bonding

Each of these is discussed below:

        1.    Dipole to Dipole Attraction:

                Remember that a Polar Molecule is a molecule in which there is a difference in the    

                electronegativities of the atoms that are sharing the covalent bond; in fact, the shared pair of

                electrons is held closer to one atom than to the other. Within each polar molecule, then, there

                is a partial positive end ( δ+) and a partial negative end (δ -). This unsymmetrical charge

                arrangement means that the polar molecule has two distinctly different "ends" or poles; it is

                also called a dipole.

                There is an electrical attraction between the partial positive end of one polar molecule and the

                partial negative end of another. This is called the dipole-dipole force.

                Because the attraction is between partial positive and partial negative charges, the dipole -

                dipole attraction is much weaker than the attraction which exists in an ionic bond (where stronger +

                and - charges exist.)

        2.    London Forces:

                Molecules are always moving. As two molecules approach one another, the electrons in each

                of the molecules are repelled by the electrons in the other. Because the electrons within any

                given molecule are in constant motion, the electrons, when repelled, move away from the

                electrons in the nearby molecule. This momentary distortion means that each of the

                approaching molecules becomes temporarily polar (because each one has partial positive and

                partial negative charges.) There is therefore a dipole to dipole attraction between these

                temporarily polar molecules.

                London forces occur between all molecules; they also occur between all atoms and all ions. In

                fact, London forces would be the only attraction between the inert gases (example: H₂.)

                London forces are very weak--only about 1/1000 of the strength of a covalent bond.

        3.    Hydrogen Bonding:

                A Hydrogen Bond is similar to a Dipole-Dipole Force, but it is stronger.

                A Hydrogen Bond exists between molecules in which Hydrogen is bonded to a small, extremely

                electronegative atom (F, O, or N.) Hydrogen bonds, therefore, occur between molecules

                which have a H to F, H to O, or H to N bond. Because the F (or O or N) have such

                extremely high electronegativity values, the partial positive (δ+) and partial negative (δ-)

                charges are relatively large. The attractions between these molecules are therefore much

                stronger than those involved in regular dipole-dipole forces.

                Question:    List attractive forces that exist between the following molecules:

                   (a) N₂    (b) CH₄    (c) H₂O    (d) CHCl₃    (e) Br₂

                Answer:       (a)    London forces

                                    (b)    London forces*

                                    (c)    London forces and hydrogen bonding

                                    (d)    London forces and dipole-dipole forces

                                    (e)    London forces

*Remember that VSEPR rules show that the CH₄ molecule has a tetrahedral shape and is therefore nonpolar.

Effects of Hydrogen Bonding:

Compounds whose molecules are attracted and held by hydrogen bonds have a special set of properties,

and these properties are caused by the fact that it is difficult to separate the molecules. Water is a good

example of a substance that exhibits hydrogen bonding, and the properties of water that are caused by

hydrogen bonding between water molecules are as follows:

        1.    Water molecules attract one another (by hydrogen bonding) and therefore tend to form chains

               and clumps of molecules.

      2.   Water has a high surface tension. The water molecules on the surface are attracted and held

            to one another; in addition they are attracted and held by the water molecules beneath the

            surface. For this reason, water molecules do not escape (evaporate) easily from the surface.

            That is why water has a high surface tension.

      3.    Water has a high vapor pressure. Its high surface tension tends to keep water molecules from

             evaporating readily.

        4.    Water has a higher boiling pint than its low molecular weight would indicate.

        5.    Water has an exceptionally high heat content (specific heat).

               (Note: Since it takes a lot of heat to break the molecules of water apart, water has to absorb

               more heat energy in order to change temperature or physical state.)

        6.    Water has a higher melting point than its weight would indicate.

        7.    Water expands when it freezes. Its point of greatest density is 4⁰C.