Interview with a Crystal Lattice Construction Crew

STANDARDS CORRELATION
California Content Standards for Chemistry: Standard #2 - Chemical Bonds
Biological, chemical, and physical properties of matter result from the ability of atoms to form bonds from electrostatic forces between electrons and protons and between atoms and molecules. As a basis for understanding this concept, students know salt crystals, such as NaCl, are repeating patterns of positive and negative ions held together by electrostatic attraction. (Standard 2c)

STUDENT LEARNING OBJECTIVES
1. Students will demonstrate their understanding of how cations and anions are arranged in an ionic compound by using modeling clay and toothpicks to construct an accurate model of the crystal lattice structure of sodium chloride (NaCl).

2. Students will demonstrate their understanding of the nature and importance of the crystal lattice structure of ionic compounds by participating in videotaped interviews about the nature of ionic bonds and ionic compounds.

OTHER OBJEVTIVES
Video clips of each group¡¯s interview will be evaluated and then posted by the teacher on the class¡¯ Moodle website. In addition to being used to evaluate student work more accurately and thoroughly than would otherwise be possible, the video clips will be used to 1) promote parent involvement in the classroom by enabling parents to observe the lesson and discuss it with their children, and 2) support student success by providing students who were absent on the day of the activity the opportunity to observe and learn from it.

TIME REQUIRED
1-2 class periods, 50-minutes each

LESSON OVERVIEW
*Prior Knowledge
At this point in the semester, students know the definitions of ¡°atom¡±, ¡°ion¡±, ¡°cation¡± and ¡°anion¡±. They know the element names and symbols, how and why ions are formed, the charge that atoms in each group on the periodic table tend to form when they become ions, and periodic trends in the sizes of atoms and ions. They also know what a chemical bond is, why atoms form bonds, and which electrons are involved in bonding. Finally, they know that there are three types of chemical bonds ¨C ionic, covalent, and metallic ¨C and that substances with different types of bonds have different physical properties. The lesson will begin with a brief review activity designed to activate students¡¯ prior knowledge.

*Direct Instruction
Students will listen to a 15-20 minute multimedia lecture that includes the following topics:
- transfer of electrons as the basis of ionic bonding,
- the relative sizes of cations and anions,
- how ionic compounds are formed through electrostatic attraction,
- why cations and anions arrange themselves into a crystal lattice structure,
- why the crystal lattice structure is so strong,
- the physical properties of ionic compounds that result from the crystal lattice structure,
- what empirical formulas reveal about ionic compounds

*In-Class Activity Part 1 (40 points possible)
Students will work in groups of four to create a 5x5 model of the crystal lattice structure of NaCl using modeling clay and toothpicks.

Evaluation: The models must use different colors and sizes to represent the cations and anions. The model must consist of a repeating pattern of alternating cations and anions with toothpicks representing the ionic bonds between them. All group members must participate in the construction of the model to receive full credit for this part of the activity.

* In-Class Activity Part 2 (60 points possible)
Students will use video cameras to conduct interviews of their group members on a rotating basis. (Sample questions and answers are provided below.) While each question will be directed toward a specific group member, all group members may discuss each question and help formulate a response prior to moving on to the next question. Groups will be evaluated on the number of correct responses according to the rubric below.

Evaluation: Students¡¯ video clips will be reviewed by the teacher and scored according to the following rubric:

# Correct Responses Points Earned
11-13 60 pts
9-10 50 pts
7-8 40 pts
5-6 30 pts
¡Ü4 ¡Ü20 pts

Sample Interview Questions and Answers:

1. What are you building?
- A model of the crystal lattice structure of the ionic compound, sodium chloride.

2. Which is the sodium ion and which is the chloride ion?
- The smaller sphere is the sodium ion and the larger is the chloride ion.

3. Why is chloride bigger than sodium?
- Chloride is the anion. It gained electrons to become an ion so it is larger. Sodium is the cation. It lost electrons to become an ion so it is smaller.

4. How are ionic bonds formed?
- Atoms don¡¯t exist as ions in nature. They have to be around other atoms that make them ¡°want¡± to become ions. For example, the neutral chlorine atom has 7 valence electrons. It needs 8 valence electrons to be stable. The neutral sodium atom has 1 valence electron. It also needs 8 valence electrons to be stable. When neutral sodium and chlorine atoms come near each other, chlorine ¡°steals¡± sodium¡¯s one valence electron. This gives chlorine a full valence shell and a negative charge. It also gives sodium a full valence shell and a positive charge (after losing the 1 electron in its outer shell, that shell disappears, revealing a new valence shell beneath with 8 electrons). The positively charged sodium is then attracted to the negatively charged chloride and an ionic bond is formed.

5. Why is your structure arranged so that there is never a cation next to another cation or an anion next to another anion?
- Opposite charges attract and like charges repel. The sodium cation and chloride anion are attracted to each other but two sodium cations or two chloride anions next to each other would repel.

6. What do the toothpicks represent?
- The toothpicks represent the ionic bonds between the cations and anions.

7. Why is the crystal lattice structure so strong?
- The repeating pattern of alternating (+) and (-) ions minimizes repulsions between similarly charged ions and maximizes attractions between oppositely charged ions.

8. What does the empirical formula reveal about an ionic compound?
- It tells you the ratio of ions (as opposed to the actual number of ions in the compound). For example, in the ionic compound sodium chloride, there is one sodium ion for every one chloride ion. Therefore the empirical formula is NaCl even though there may be millions of each ion in any given sample of NaCl.

9. How does the crystal lattice structure explain why ionic compounds are brittle crystalline solids at room temperature?
- A repeating pattern of unit cells with a crystal lattice geometry at the microscopic level translates into crystalline appearance at the macroscopic level. Ionic compounds are brittle because they can shatter along the lines of the unit cells.

10. How does the crystal lattice structure explain why ionic compounds have such a high melting point?
- Ionic bonds are very strong and all of them must be broken for an ionic compound to melt. This takes a lot of heat energy, thus the melting point is very high.

11. How does the crystal lattice structure explain why ionic compounds are highly soluble in water?
- Ionic compounds are made of ions. Polar water molecules are able to surround and separate cations and anions individually thereby dissolving the compound.

12. How does the crystal lattice structure explain why ionic compounds conduct electricity in solution?
- In order to conduct electricity, ions must be free to travel and conduct electric current. When ionic compounds are dissolved, the cations and anions move freely in the solution.

13. Why don¡¯t ionic compounds conduct electricity in their solid form?
When in their solid form, ionic compounds contain cations and anions but they are locked in place. They are not free to travel and conduct electric current.

* At-Home Activity (20 points possible)
1. Students will complete the Worksheet ¡°Ionic Bonds and Ionic Compounds¡± with a score of 80% or better.

2. Students will log on to the class Moodle site to view and discuss their video with their parent or guardian. Parents will sign the bottom of the worksheet to verify that this has occurred.