Density Is a Periodic Property

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!

Chemical Concepts:

•Periodic law               •Density          • Group 14 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 14 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 14 column of elements. In 1871, Mendeleev proposed that there existed an as-yet-unknown element beneath silicon in the Group 14 elements. He named the missing element ekasilicon 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 ekasilicon 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 4 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:

Pre-Lab Questions:

1. One of the elements Mendeleev predicted was ekaaluminum, corresponding to a gap in the fourth row or period of the Group 13 elements, between aluminum and indium. The density of aluminum (period 3) is 2.70 g/cm3, that of indium (period 5) 7.31 g/cm3, 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 ekaaluminum. What known element in the modern Periodic Table corresponds to ekaaluminum? 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? Calculate the percent error between the predicted and actual values for the density of ekaaluminum.

Materials:

Lead shot, Pb, 35 g                                         Paper towels

Silicon lumps, Si, 8 g                                      Water

Tin shot, Sn, 25 g                                            Balance, centigram (0.01 g precision)

Beakers, 50-mL, 3                                           Graduated cylinder, 25 mL

Forceps or tongs                                              Marking pencil or pen

Tape

 

Safety Precautions:

Lead powder is extremely toxic by inhalation and ingestion, lead fumes and 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. 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 mass, 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 mass, final mass and volume data in the Data Table.

15. Return the correctly labeled solids to your instructor for reuse.

Data Table:

Element

Sample

Initial

Mass

(g)

Final

Mass

(g)

Mass of

Solid

(g)

Initial

Volume

(mL)

Final

Volume

(mL)

Volume of

Solid

(mL)

Silicon

1

 

 

 

 

 

 

2

 

 

 

 

 

 

3

 

 

 

 

 

 

Tin

1

 

 

 

 

 

 

2

 

 

 

 

 

 

3

 

 

 

 

 

 

Lead

1

 

 

 

 

 

 

2

 

 

 

 

 

 

3

 

 

 

 

 

 

 

Post-Lab Calculations:

1. Complete the Data Table: Calculate both the mass (initial mass -final mass) and volume (final volume -initial volume) for each trial.  Record these results in the Data Table.

2. It would be advisable to add to the table above or construct a new table for the information from your calculations.  The table should include density for each trial, an average density for each element and a plus or minus value for each average density. Note: The density of a solid is usually reported in units of g/cm3.

3. Using the mass and volume data, calculate the density of each tria.

4. Calculate the average value (mean) of the densities 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., if your densities are 6.8 g/cm3, 7.0 g/cm3 and 7.2 g/cm3 your average would be 7.0 g/cm3.  Your plus or minus value would be ±0.2 g/cm3).

5. 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.

6. Look up the actual density of germanium in a reference source and calculate the percent error between the predicted and actual values (see Pre-Lab Question #3).