Materials Included In Kit

Additional Materials Required

Safety Precautions

Lead powder is extremely toxic by inhalation and ingestion; lead fumes or dust are possible carcinogens. Using lead shot does not present a powder or dust hazard. Take all precautions to avoid obtaining lead powder. Silicon is flammable in powder form and is slightly toxic by ingestion. Avoid grinding the silicon chunks when the element is dried or recycled. Do not breathe or handle any fine silicon powder remaining on the bottom of the reagent bottle. Wear chemical splash goggles and chemical-resistant gloves and apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All of the element samples should be thoroughly dried and stored for repeat use. Avoid excessive handling of the silicon lumps. Do not pulverize the material.

Teacher Tips

• This kit as a Super-Value kit—all of the chemicals can be dried and recycled for future use. The lab work can reasonably be completed in one 50-minute lab period.
• Prior to lab, review with students the idea of significant figures and remind them to include the appropriate number of decimal places in their volume readings. Since minor scale divisions on a 25-mL graduated cylinder are marked every 0.2 mL, volume measurements can be estimated to 0.1 mL.
• The sample masses of silicon, tin, and lead have been chosen to allow two significant figures (±0.1 mL) in the volume measurements and to keep the final water volume below the 25-mL capacity of the graduated cylinder. Larger graduated cylinders (100-mL) may also be used, but the sample masses should be increased in order to obtain reliable precision in the volume measurements.
• If the element samples must be reused for several class periods in one day, it will be helpful to provide a way to dry the solids quickly. This can be done by gently heating the solids using a blow dryer or in a pan on a hot plate.
• The “best-fit” straight line should include as many of the data points as possible. This lab provides excellent data to introduce or reinforce the use of graphing calculators and/or graphical analysis programs.

Further Extensions

Extensions

• Challenge students to suggest other properties of the elements, in addition to period number, that can be used to predict the density of germanium within the same group of elements. For example, density can be graphed against either atomic mass or atomic number. Since atomic masses were known at the time of Mendeleev, this reasoning is “fair” and in keeping with how Mendeleev might have predicted the properties of his missing elements. Atomic mass gives a less accurate prediction of the density of germanium than does the period number. Correlation of atomic number with density actually gives the most accurate prediction of the density of germanium. At the time of Mendeleev, of course, there was no concept of atomic number, since protons had not yet been discovered (and would not be for another 40 years or so). Sample graphs of density versus atomic mass and atomic number are shown in the Supplementary Information section.

Answers to Prelab Questions

1. One of the elements Mendeleev predicted was eka-aluminum, corresponding to a gap in the fourth row or period of the Group IIIA elements, between aluminum and indium. The density of aluminum (period 3) is 2.70, that of indium (period 5) 7.31 and that of thallium (period 6) 11.85 g/cm³. Make a graph of period number on the x-axis versus density on the y-axis for each of these elements.
   {12015_PreLabAnswers_Figure_2}
2. Use your graph to predict the density of eka-aluminum. What known element in the modern Periodic Table corresponds to eka-aluminum? Look up the density of the modern element in a reference source and record its actual and predicted density values.
   Using the “best-fit” straight line through the data, the predicted density of the Group III element in period 4 of the Periodic Table would be about 5.3 g/cm³. Gallium is the modern element corresponding to eka-aluminum, whose existence and properties were predicted by Mendeleev. The actual (literature) density of gallium is 5.90 g/cm³.
3. How do the actual and predicted density values compare? Use the following equation to calculate the percent error between the predicted and actual values for the density of eka-aluminum.
   {12015_PreLabAnswers_Equation_2}
   The predicted density is less than the actual value.
   {12015_PreLabAnswers_Equation_3}

Sample Data

Answers to Questions

1. Complete the data table: Calculate both the mass (initial mass – final mass) and volume (final volume – initial volume) of each sample 1–3 for all three elements, silicon, tin and lead. Record these results in the data table.

   See the Sample Data section.

2. Using the mass and volume data, calculate the density of each sample 1–3 for all three elements. Construct a results table to summarize the results. Note: The density of a solid is usually reported in units of g/cm³. Recall that 1 mL = 1 cm³.

   Results Table

   {12015_Answers_Table_2}
3. Calculate the average value (mean) of the density calculations 1–3 for each element, silicon, tin and lead. Record all results in the results table. Use the range of density values for each element to estimate “plus-or-minus” (±) error for each average (e.g., 7.0 ±0.2 g/cm³).

   See results table.

4. On a graph, plot the period number of Si, Sn and Pb on the x-axis versus the average density of each element on the y-axis. Using a ruler or straightedge, draw a “best-fit” straight line through the data points. Use this “best-fit” straight line to predict the density of germanium.

   See the sample graph. Drawing a horizontal line from the period number of germanium (4) across to the y-axis gives a predicted density of germanium equal to 5.0 g/cm³.

   {12015_Answers_Figure_3}
5. Look up the actual density of germanium in a reference source and calculate the percent error between the predicted and actual values (see Prelaboratory Question 3).

   The literature density of germanium (from the CRC Handbook of Chemistry and Physics) is 5.32 g/cm³.

   {12015_Answers_Equation_4}

Introduction

Dmitri Mendeleev proposed the periodic law for the classification of elements in 1869–1871. After observing trends in the properties of elements when they were arranged in order of increasing atomic mass, Mendeleev made a startling prediction. He predicted the existence and properties of at least three undiscovered elements. Mendeleev saw what other scientists before him had missed—he saw what wasn’t there!

Concepts

• Periodic law
• Density
• Group IV elements
• Period number

Background

At the time Mendeleev proposed the periodic law, the foundation of the modern periodic table for the classification of elements, 63 elements were known. Their physical and chemical properties had been studied and their atomic masses measured. Mendeleev arranged the known elements in a calendar-like table of rows and columns in order of increasing atomic mass and repeating chemical properties. It is at this point, however, that Mendeleev made a giant leap of discovery—he suggested that there were some gaps or missing elements in the list of known elements.

Among the Group IV elements in Mendeleev’s classification scheme, carbon appeared in the second row, followed by silicon in the third row. Both tin and lead shared similar chemical properties with carbon and silicon and were also known at this time. Because of their high atomic masses, however, these metals were placed in later rows of Mendeleev’s Group IV column of elements. In 1871, Mendeleev proposed that there existed an as-yet-unknown element beneath silicon in the Group IV elements. He named the missing element eka-silicon and predicted its physical properties (atomic mass, melting point, density, and specific heat). In 1886 the element germanium was discovered by the German chemist Clemens Winkler. In his report of the discovery, Winkler stated: “...There can be no longer any doubt that the new element is no other than the eka-silicon prognosticated fifteen years ago by Mendeleev.”

Within 15 years of Mendeleev’s prediction of the existence of missing elements, three of the elements had been discovered, their properties in excellent agreement with those predicted by Mendeleev. Is it possible to recreate some of the excitement that followed the prediction and discovery of Mendeleev’s missing elements?

Experiment Overview

The purpose of this experiment is to measure mass and volume data for silicon, tin, and lead, calculate their densities, and use these results to predict the density of germanium, Mendeleev’s “undiscovered” element in the Group IV family of elements. The volume of the elements will be measured by water displacement (see Figure 1).

Measuring the Volume of a Solid by Water Displacement

Materials

Prelab Questions

1. One of the elements Mendeleev predicted was eka-aluminum, corresponding to a gap in the fourth row or period of the Group IIIA elements, between aluminum and indium. The density of aluminum (period 3) is 2.70, that of indium (period 5) 7.31, and that of thallium (period 6) 11.85 g/cm³. Make a graph of period number on the x-axis versus density on the y-axis for each of these elements.
2. Use your graph to predict the density of eka-aluminum. What known element in the modern Periodic Table corresponds to eka-aluminum? Look up the density of the modern element in a reference source and record its actual and predicted density values.
3. How do the actual and predicted density values compare? Use the following equation to calculate the percent error between the predicted and actual values for the density of eka-aluminum.
   {12015_PreLab_Equation_1}

Safety Precautions

Lead powder is extremely toxic by inhalation and ingestion; lead fumes or dust are possible carcinogens. Using lead shot does not present a powder or dust hazard. Do not work with lead powder. Silicon is flammable in powder form and is slightly toxic. Do not breathe or handle any fine silicon powder remaining on the bottom of the reagent bottle. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash your hands with soap and water before leaving the laboratory.

Procedure

1. Label three 50-mL beakers or small containers Si (silicon), Sn (tin) and Pb (lead).
2. Obtain approximately 8 g of silicon chunks in the appropriately labeled beaker. Measure the combined mass of the beaker plus solid to the nearest 0.01-g and record the value in the data table. (Note: This value is the initial mass for sample 1.)
3. Fill a 25-mL graduated cylinder approximately half-full with water. Measure the initial volume of water and record the value to the nearest 0.1 mL in the data table.
4. Using forceps or tongs, carefully add about one-third of the silicon chunks to the graduated cylinder (enough to raise the water level in the cylinder by at least 1.0 mL). Add the solid slowly, so as to avoid splashing or breaking the glass cylinder.
5. Measure and record the new (final) volume of water plus solid in the graduated cylinder.
6. Measure and record the combined mass of the labeled beaker and remaining solid in the data table. (Note: This value is the final mass for sample 1.)
7. Repeat steps 4–6 twice with some of the remaining amount of solid in the beaker. Do NOT empty the graduated cylinder between samples. The final volume of the previous sample becomes the initial volume for the next sample.
8. Record all initial and final mass and volume data in the data table. There should be a total of three sets of mass and volume data (samples 1–3).
9. After all three trials have been completed, empty the water from the graduated cylinder. Carefully pour all the silicon chunks onto a paper towel and allow them to dry. Do not allow any of the solid to go down the drain.
10. Rinse the graduated cylinder with water.
11. Obtain approximately 25 g of tin shot in the appropriately labeled beaker. Measure the initial mass of the beaker plus solid to the nearest 0.01 g and record the value in the data table.
12. Repeat steps 3–10 using tin. Record all initial and final mass and volume data in the data table.
13. Obtain approximately 35 g of lead shot in the appropriately labeled beaker. Measure the initial mass of the beaker plus solid to the nearest 0.01 g and record the value in the data table.
14. Repeat steps 3–10 using lead. Record all initial and final mass and volume data in the data table.
15. Return the correctly labeled solids to your instructor for reuse.

Student Worksheet PDF

12015_Student1.pdf