I. SOLID STRUCTURES AND PROPERTIES—Variations in bond type lead to different structures in solids.
A. GIANT ATOMIC STRUCTURES—These are structures where atoms are bonded to one another with covalent bonds in a massive (or giant), continuous network. Examples include diamond and graphite.
1. Diamond is based upon a tetrahedral unit where all of the carbon atoms are bonded to four others with very strong covalent bonds in a huge macrostructure.
i. Large numbers of strong covalent bonds makes diamond very strong and hard and gives it a high melting and boiling point.
2. Graphite has a layered structure where each carbon atom is covalently bonded to three others in each plane. The structure leads to specific properties:
i. It will conduct electricity only in one plane. In the graphite structure, each carbon is only bonded to three others. This leaves one of each carbon atoms’ valence electrons “free.” These electrons are spread out over each layer (delocalized). This leads to a “sea” of electrons similar to that in the metallic structure and is responsible for graphite’s ability to conduct electricity along the layers. Because the electrons cannot move between layers, there is no conduction from one layer to another.
ii. It can be used as a lubricant. Weak London dispersion forces (see Chapter 9) hold the layers in graphite together. As a result, they can slide over one another, making graphite a good lubricant.
B. IONIC STRUCTURES—These are structures where a giant lattice of ions is held together by strong electrostatic interactions (ionic bonds) between the opposite charges. An example is sodium chloride.
1. The strong ionic bonds give ionic solids high melting and boiling points.
2. They can only conduct electricity when molten or in solution because when they are in the solid state, the ions are rigidly held and cannot move. When molten or in solution, the ions are free to move so electricity can be conducted.
3. The charged particles present in an ionic solid explain how and why ionic solids dissolve in water. When an ionic solid dissolves, the polar water molecules penetrate the lattice and attach themselves to the ions. The process is called hydration and the ions are said to be hydrated.
You may have heard the phrase “like dissolves like”; this refers to the fact that charged substances (like ionic solids) tend to dissolve in polar (charged) solvents. Remember the phrase—it’s a good rule of thumb.
C. MOLECULAR STRUCTURES—These are structures where molecules are attracted to one another with weak intermolecular forces. An example is iodine.
1. Weak intermolecular forces hold the molecules in place in the solid.
2. Weak forces mean that molecular solids tend to have very low melting points because there is only a weak interaction between its molecules, and they tend to be soft.
3. No charged particles are present, making molecular solids nonconductors.
Most test questions expect you to be able to relate observed physical properties such as melting and boiling points, electrical conductivity, hardness, and water solubility to the type of structure and bonding present. Focus your review on the relationships between structure and properties.