Concept/Skills Development

16. Metals have characteristic properties that can be explained in terms of bonding. Briefly describe the bonding in metals that explains such properties. [Student answers will vary. The basic ideas are found in the post-lab discussion for Laboratory Activity 1 involving chemical bonding in solids.]

17. Some substances are molecular with low melting points, soft structure, and low solubility in water. What kind of bonding could account for these properties? [Student answers will vary. This is a very open-ended question; it's probable that students will not give complete answers without help from you. Basically, three types of intermolecular interactions-not including the strong covalent bonding within the molecules themselves-may be responsible. These interactions are collectively referred to as van der Walls forces. The weakest intermolecular forces are London forces, as seen in noble gases, carbon dioxide, and other non-polar, low-melting substances. These forces can be viewed-at least at a basic level-as due to motion of electrons and formation of temporary dipoles. The forces become stronger as the total number of electrons within molecules and the surface are of the molecules increase. For example, carbon dioxide melts at a considerably higher temperature than does molecular hydrogen.

Another type of weak intermolecular bonding force is a dipole-dipole force. Two kinds of these forces, distinguished by the energy required to break them are possible. One is the attractive force between opposite charges of polar molecules. It is stronger than London forces but much weaker than ionic forces in ionic solids. Molecular solids and liquids with this type on bonding generally have higher melting points.

Some compounds appear to have abnormally high melting points when compared to compounds of similar size, shape, and total electrons. In many of these cases, such compounds exhibit hydrogen bonding, involving highly electronegative nitrogen, oxygen, or fluorine atoms. The small size and high electronegativity of such atoms cause highly unequal sharing of the electron pair forming the covalent bond to hydrogen; there is substantial separation of charge. The intermolecular attractions (between the hydrogen atom and a lone pair of electrons on a N, O, or F atom from an adjacent molecule) arising in this fashion are about an order of magnitude stronger than ordinary dipole-dipole bonding. Water is an example of a hydrogen-bonded substance. If its intermolecular forces were simple dipole-dipole forces, it would melt and boil at lower temperatures than hydrogen sulfide, H2S, a gas at room temperature.]

Pictures in the Mind

  1. Close Encounters-Sticky Collisions: A Group Activity
    Have six-student groups, wearing placards hanging from their neck, act out the following scenario while you describe the action. After one run-through, have the entire class practice this (in groups of six students) to get a feeling for bonding. This can also be reinforced using appropriate cut-outs on a flannel board or on the overhead projector. Molecules form and chemical reactions can occur when atoms "collide." There are three important types of collisions. These are:

    Close Encounters of the First Kind: Sighting
    An interaction between two atoms in which nothing happens-not a reactive collision-often called "scattering." (Students acting this out should be given directions in advance about how to simulate Close Encounters of the First Kind.)

    This is the reaction of a hydrogen atom Ha with a hydrogen molecule Hb-Hc. Ha collides with the molecule. If Ha sticks to Hb, then Hc will be ejected from the other end and a new molecule, Ha-Hb will result. Sighting (CEFK), however, is a weak interaction of Ha with Hb but no sticking occurs; Hb-Hc remains intact.

Chemical Bonding (BOND)
Page 31