Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Form a working group as directed by your teacher and obtain all of the necessary materials, including the Plotting Trends Worksheet and two handouts, “Representative Element Data” and “Periodic Placement of the Elements.”
2. The “Representative Element Data” handout lists the properties of the first 34 members of the representative or main group elements (Groups IA–VIIIA). The transition metals are not included in the list.
3. Your teacher will assign each group one physical property to study.
4. In the calculations that follow, one cm is first subtracted from the maximum straw length (step 4) to determine a “straw scale” ratio. This is then added back on to determine the cut straw length for each element (step 7). One centimeter is the depth of the wells on the reaction plate.

Teacher Tips

• Make enough copies of the Plotting Trends Worksheet and the Representative Element Data and Periodic Placement of the Elements handouts to give all students individual copies. Two 50-minute class periods will likely be needed to complete this activity. The activity may be completed in one class period if the “straw length” calculations are assigned as homework.
• This kit contains enough materials for a class of 24 students working in groups of four (six student groups). Six physical properties of the elements are listed on the Representative Element Data handout. This makes it possible to assign each group of four in a class of 24 students a unique physical property to study. All physical properties on the handout will be accounted for and there will be no duplications. For smaller or larger class sizes, adjust the sizes of the working groups and discard or double up on the assigned physical properties, as needed.
• The Representative Element Data handout lists the physical properties of 34 representative (main group) elements—Groups IA–VIIIA and periods 1–5 in the periodic table. The transition elements are not included. Notice the gaps on the handout for atomic numbers 21–30 and 39–48.
• This is best performed as a cooperative group activity. Monitor or assess student interaction to assure that students share responsibilities and decisions. It may be helpful to suggest that one student in each group take on the role of checking all calculations.
• Review beforehand or assign as prelab preparation the definitions and units of the physical properties.
• Set aside a common area for students to display their charts and the accompanying descriptive index cards. Students may report on their investigation and lead a discussion of possible explanations for the observed periodic trend.
• The reaction plates and handout masters are reusable from year to year. Save all materials for future use. The only consumable items that must be resupplied are straws!
• See the Sample Data section for an illustration of a possible “straw chart” and an explanation of the observed periodic trend in the ionization energies of the elements.

Sample Data

Ionization Energy

Definition: Energy needed to remove one of the electrons from an atom (specifically, from an isolated atom in the gas phase). The energy needed to remove successive electrons is called the first ionization energy, second ionization energy, etc. Units are kilojoules per mole (kJ/mole).

Periodic Trend: Ionization energy increases going up a column and from left to right in the periodic table. Thus, among the naturally occurring, representative (main group) elements, helium has the highest ionization energy and cesium the lowest. Note: There is a slight dip in the ionization energy between the Group IIA and IIIA elements as one proceeds from left to right across any period in the periodic table (see Figure 2).

Explanation: Ionization energy decreases as the period number increases because electrons are held less strongly the further out they are from the nucleus. Recall that the period number for any element in the periodic table corresponds to the value of the principal energy level for its outermost electrons. The distance from the nucleus increases as the value of the principal energy level increases. Ionization energy increases from left to right across the periodic table because the effective nuclear charge (number of protons) of an atom increases in this direction, while the approximate distance of the electrons from the nucleus remains the same. Thus the electrons are held more strongly and require more energy to remove. The apparent “zig-zag” pattern in the ionization energies of the Group IIA and Group IIIA elements is due to the difference in energy between the ns and np orbitals. The p-orbitals are slightly higher in energy than the s-orbital and the p-electrons are thus easier to remove.

Teacher Handouts

12048_Teacher1.pdf

12048_Teacher2.pdf

Introduction

A wealth of numerical data is available in most textbooks concerning the properties of elements—their ionization energies, atomic radii, electronegativity values, etc. What do all the numbers mean? Visualizing trends from the numerical data can be difficult. In this activity, we will use reaction plates and straws of different lengths to construct three-dimensional models of the physical properties of the elements and then examine them for trends.

Concepts

• Periodic table
• Periodic trends
• Ionization energy
• Atomic radius
• Electronegativity

Materials

Safety Precautions

Although the materials in this activity are considered nonhazardous, follow all normal laboratory safety guidelines.

Procedure

1. Locate your assigned physical property on the Representative Element Data handout and find the maximum value of this property for the elements listed. Example: The maximum value of the density for the elements listed is 7.31 g/cm³ (for tin).
2. Measure the length of one straw in centimeters to the nearest 0.1 cm.
3. Subtract one cm from the actual straw length. Let this value represent the maximum value of the assigned physical property. Example: For a straw that is 19.5 cm long, a straw length of 18.5 cm will represent a density of 7.31 g/cm³.
4. Divide this straw length by the maximum value of the physical property to calculate a “straw scale” ratio for the assigned physical property. Example: The “straw scale” ratio for density is equal to (18.5 cm)/(7.31 g/cm³) = 2.53 cm/g/cm³.
5. Use the “straw scale” ratio and Equation 1 to calculate a straw length for each element in the list based on the value of the assigned physical property for that element. Round off all straw length calculations to 0.1 cm. Record the “calculated” straw lengths on the worksheet.
   {12048_Procedure_Equation_1}
   Example: The density of beryllium is 1.85 g/cm³. Solving Equation 1 for the straw length shows that a calculated straw length of 4.7 cm is needed to represent the density of beryllium.
   
   

   straw length (Be) = 2.53 cm/g/cm³ x 1.85 g/cm³ = 4.7 cm

6. Add 1 cm to the calculated straw length for each element. This will represent the length that each straw must be cut. Record the “cut” straw lengths on the Plotting Trends Worksheet.
7. Cut a straw to this length for each element. Example: Cut a straw 5.7-cm long to represent the density of beryllium.
8. Place the cut straw in the reaction plate to match the position of the element in the periodic table (see the Periodic Placement of the Elements handout). Example: Beryllium is in period 2 (the second row) and Group IIA (the second column). It should be placed in the second row, second column of wells in the reaction plate (see Figure 1).
   {12048_Procedure_Figure_1_Placement of elements on the reaction plate}
9. (Optional) Place a pea-size amount of plumber’s putty or clay in the bottom of each straw before placing it in the reaction plate. This will make the resulting “straw chart” more stable.
10. Examine the three dimensional “straw chart” from top to bottom and from left to right. Identify any trends or periodic relationships that exist for your assigned physical property.
11. Create a descriptive card to be displayed along with the three-dimensional “straw chart.” Include the following information on the index card:
    • Names of group members
    • The assigned physical property of the elements
    • Description of the observed periodic trend
    • Proposed explanation for the observed trend.

Student Worksheet PDF

12048_Student1.pdf