Chapter 1. Symbolic Descriptions

• DEMO: Test solubility and conductivity
• Begin by testing the solubility of the solids. Next test the solubility of the liquids. Then test which substances dissolve in water. For those that dissolve, test the solution conductivity.
• [Note: Technically, the drawing is incorrect because you need ac current.]

• Ask students to record data.
• Give students an opportunity to look for patterns.
• There are four different yes/no sequences; yes/no/no, no/yes/no, no/yes/yes, and no/no/no.
• Ask students if any of the properties are mutually exclusive (if the substance conducts as a solid, it does not dissolve)

These simple experiments allow us to categorize these substances into the following four groups:

1. Dissolve in water and solutions conduct.
2. Dissolve in water and solutions do not conduct.
3. Do not dissolve in water but conduct as solids.
4. Do not dissolve in water and do not conduct as solids.

The differences in physical properties can be accounted for by differences in bonding.

Note: There are exceptions to these generalizations. For example, some molecules form solutions that conduct electricity (e.g., HCl), and not all salts are soluble. We want to explore the generalizations first, and then refine our thinking by examining the exceptions.

We want to consider various ways in which the atoms that make up the solids might be arranged and how they might be held together.

Packing of spheres/atoms:

• The oranges pack regularly. The grapes are a disordered mixture. The beads are woven together into thin flexible strips.
• Atoms in solids can be ordered in three D arrays. Atoms of two different sizes can pack well together with the smaller atoms in the spaces created by packing the larger atoms. Atoms in fats are attached in chains.
• Other possibilities?

How can you make spheres stick together? (coat with glue, put sticks/thread/velcro/glue in specific directions, magnets)

DEMO: Packing of small magnetic beads.

This class of compounds is characterized by the classical covalent bond where the octet rule is satisfied by sharing electrons between nonmetal atoms. The bonds are confined to individual molecules.

DEMO: Show dry ice (solid CO₂)

Properties: It is soft, you can crush it, it evaporates

This class of compounds is also characterized by the classical covalent bond. However, the bonds link atoms together extending in 1D, 2D, or 3D “supermolecular” networks or grids.

DEMO: Show ceramic crucible (Al6Si2O13 - mullite)

Properties: hard (resists wear), mechanical strength in spite of brittleness

On the board:

• Use the octet rule to determine the structures of Si and [CH₂]_n.
• Which of these do you expect is the hardest: Si or [CH₂]_n? Explain.

Use the octet rule to determine the structures of C and [CH₂]_n?

Which of these do you expect is the hardest: C or [CH₂]_n? Explain.

SiO₂ is a network covalent solid. Although C is just above Si on the periodic table, CO₂ (dry ice) is a molecular solid. How do the structures of CO₂ and SiO₂ differ?

Transfer of electrons from the metal to the nonmetal. The metal becomes a cation with zero electrons in the valence shell. The nonmetal becomes an anion with 8 electrons in the valence shell. The total charge adds to zero

Transfer of electrons from the metal to the nonmetal. The metal becomes a cation with zero electrons in the valence shell. The nonmetal becomes an anion with 8 electrons in the valence shell. The total charge adds to zero

Ions arrange in a well-order crystal lattice. Cations and anions are attracted to one another (opposite charges attract).

Coulomb’s law: The attractive force is proportional to the charges, and inversely proportional to the square of the distance between the charges.

Higher charges = greater attractions
Smaller sizes = greater attractions

DEMO: Show Rock salt, NaCl

Properties: Hard, brittle, dissolves in water

The locations of the cations and anions maximize attraction and minimize repulsion.

Same group = same charge

The magnitudes of the charges increase towards the middle of the row.

Anions have same total number of electrons as noble gas at the end of the row they are in. Cations have same total number of electrons as noble gas at the end of the previous row.

There are not enough electrons for the metal atoms to share to gain an octet. Instead the electrons simply go free so that the metals atoms become cations with zero valence electrons

Metal ions with a sea of electrons. In effect, the electrons are the anions. There is an attraction between the metal cations and the electrons.

Show example: Aluminum foil
Flexible, ductile, malleable, conducts electricity, melts at relatively low T

http://www.explainthatstuff.com/alloys.html

Hardness

• Examine what happens to each type of substance when it is struck.
• The hardest substance is a solid with network covalent bonding. This is because bonding in these substances is in an organized three-dimensional network.
• The softest substances are the molecular covalent substances. The atoms in these substances combine in units called molecules. It is relatively easy to push the molecules around and separate them from each other. This explains why so many molecular covalent compounds are liquids and gases.
• The most brittle substance is one with ionic bonding. It fractures when it is struck, in a smooth break between two pieces.
• Metallic substances are the most bendable. This is because the sea of valence electrons is easily distorted.

Conductivity

• Conductivity is the motion of charge, or electricity, within a substance. Metals conduct electricity because the valence electrons are free to move throughout the solid. Ionic compounds that have been dissolved in water conduct electricity because the cations and anions are free to move in the solution.
• Network covalent solids and molecular covalent substances do not conduct electricity. The charge cannot move in these substances because the electrons are “stuck” between the atoms.

Bonding can help to explain the properties of dissolving and conductivity. Ionic solids and molecular covalent substances dissolve in water. Metallic solids and covalent network solids do not.

The periodic table is a valuable tool in figuring out bonding. Use the table to determine if the elements in a compound or substance are metals, nonmetals, or both.

• Ionic compounds are made from metal and nonmetal elements.
• Metallic compounds are made from metal atoms alone.
• Network covalent and molecular covalent compounds are made from nonmetals.

Once you know the type of bonding in a substance, you can predict its properties.