THE MENDELEEV PUZZLE

Rena Benedict
Forest Lake Area Sr. High School, Forest Lake, MN
This curriculum module is designed to open an introductory high school chemistry course. The first activity uses samples of the chemical elements, magnets, and conductivity testers for students to develop skills in drawing conclusions--and in evaluating the usefulness of such conclusions. In the second activity, paralleling Mendeleev's assembly of a periodic table, students create their own classification scheme of elements using "element cards" (their real identities are disguised). This is an excellent opportunity to discuss the landmark historical development and the many ways to represent the periodic table. In the third, more open-ended activity, students learn introductory laboratory techniques and a specific format for recording observations -- reflecting the decades of work on elements that led to the discovery of periodicity.
Level: Grades 10-12
Time Frame: 9-12 standard class periods

Table of Contents
This module was developed as part of a project sponsored by SciMath-MN and The Bakken Museum and Library. Click to seeother curriculum modules using history and philosophy of science in this series.

Overview

These three activities are designed to focus on the nature and history of science. The first highlights the nature of observations, their context, and "error." The second emphasizes the challenges of "science-in-the-making" -- reaching conclusions before the answer is known. It can also show how our theories or representation reflect specific purposes. The final activity focuses on the nature of evidence and persuasive arguments from available data.

The first activity uses samples of the chemical elements, magnets, and conductivity testers. Students are first introduced to chemical elements in order to develop the process skill of drawing conclusions and evaluating the usefulness of conclusions. Rather than recording observations of elements, students must look for similiarties and differences among the elements they observe, each group of students investigating a different physical property of the same elements. Each group produces a poster for class discussion. Then students must generate conclusions which cover two or more properties of elements . To generate as many different answers as possible, students must first write conclusions individually, then as a small group. Large group evaluation of the usefulness of a conclusion focuses on whether or not the conclusion is supported by the observed properties of the elements, and whether or not it is an observation rather than a conclusion.

The second activity uses cards on which elements are identified only by a letter meant to disguise their identities. Chemical and physical properties are listed in a format borrowed from the chemistry faculties at St. John's University and the College of St. Benedict, Collegeville, Minnesota. Students create their own classification scheme in a simulation of the process by which Dmitri Mendeleev organized information into a periodic table. Two options exist: (1) teachers can focus student inquiry toward the standard grid-type form of the periodic table, or (2) students can be encouraged to create their own classification structure. In both options, students are constantly challenged to defend the criteria they use for classifying elements into groups. In the historico-investigative method, students should repeat historical experiments to determine the physical and chemical properties of the elements before classifying their properties. Mendeleev's classification used existing data and was a theoretical rather than investigative approach. His theory did however, lead him to predict the existence and properties of undiscovered elements. Their discovery provided empirical data to support his theory. A unique approach to a hands-on method of creating a Mendeleevian classification for "nuts and bolts" is proposed by Mark Volkmann in The Science Teacher (January 1996).

The third activity is more open-ended. Students are introduced to laboratory techniques and a specific format for recording observations. They mix chemicals without any preconceptions of what may occur in each procedure. From standard textbook definitions given by the instructor, they must create a criteria of their own to distinguish a physical change from a chemical change. The criteria must consist of observable evidence. The procedures chosen include some obvious physical or chemical changes, and some which can be interpreted with less certainty, creating another opportunity for students to defend their conclusions in a peer review. I believe this is a reasonable approximation to the discovery process that Mendeleev and his precursors experienced discovering the sixty-some elements that provided enough of a data base to create a useful generalization such as the periodic law. Student lab reports are evaluated not on whether or not a physical or chemical change is correctly identified, but by how successfully they defend their conclusions based on the observations they made in lab. A large group criteria is then created, hopefully, leading to some consensus. Finally, a lab performance assessment is used via laserdisc, in which students observe a new experiment, make observations, write and defend their conclusions.

Although many students have some previous knowledge of chemistry, they are not allowed to use textbooks until after the three activities are completed. The module has been piloted in a standard college prepartory course and in a course modified for at-risk learners. In both classes, the students were grouped heterogeneously by grade level and ability (no tracking).

History of Elements Classification Schemes

1817
According to Van Spronsen, the first observation of a relationship betwwen the atomic weights (equivalent weight) of elements was made by Johann Wolfgang Dobereiner, of the University of Jena. He found three-member groups of analgous element (triads), for which the equivalent weight of the middle element was the arithmetic mean of the weights of the other members of the triad. He concluded that this relationship must reflect some general principle--which he could not identify.

1827
Leopold Gmelin, University of Heidelberg, expands Dobereiner's triads--again in 1843 and 1852. He was the only other well-known chemist to study this relationship before 1850.

1845
Oliver Gibbs, Rumford professor at Harvard, classifies elements on the equivalents, isomorphism (crystalline form), combining relationships, and types of compounds.

1850
Max von Pettenkofer, Universiyt of Munich, revives Prout's hypothesis of primary matter. Prout believed that all elements were whole number multiples of hyddrogen. Hence, elements would not be indivisible, but have a smaller unit of structure. Pettenkofer expanded Dobereiner's triads. He noted that similar elements formed and arithmetic series and the difference between elements in each series was 8 or a multiple of 8.

1851
Jean Baptisite Dumas, Sorbonne, compared families of elemnts with families of organic comounds. He revived Prout's hypothesis and further refined his system to involve complicated arithmetic progresssions.

1852
Peter Kremers added to Dobereiner's triads. He surmised that four was the atomic weight of a basic element. Multiplication by an odd number yielded a metalloid, even-numbered weights related to non-metals.

1853
John Gladstone's analogies fell into three categories--elements with identical weights, those with weights that are multiples of each other, and those in triads.

1854-55
Josiah Cooke, Harvard, based his system on more than atomic weight--he used isomorphism, electronegativity, physical properties, chemical reactions. His system, like the others, failed to establish a continuous system including all the elemtns because ot the underlying idea that elements could be built up from some simpler form.

1857
Ernst Lessen, Wiesbaden, believed that all elements except niobium could fit into triads. The attempt to classify all elements was encouraging, the results difficult to justify.

William Odling constructs a grouping based on analgous properties. He finds a relationship between four of his thirteen groups.

1860
Matthew Lea, Philadelphia, finds new relationships between atomic weights, even considering negative atomic weights. He attempts to show that the sums and differences of atomic weights gave the numbers 44 or 45. He attempts to predict atomic weights for elements not yet discovered.

1862
Van Spronsen considers this to be the birth year of the periodic system--he lists six discoverers, including Mendeleev. In 1862, Alexander de Chancourtois presented a system showing periodicity as a function of atomic weight in a three-dimensional representation. His Vis Tellurique, so named because tellurium was at the center, was a spiral encompassing a cylinder (also known as the telluric screw).

1865
John Newlands constructs a system of classification based on octaves. His use of atomic weights did not reflect the precision available. His work was ridiculed as having no more basis than if he had chosen an alphabetical listing as a classification.

Odling revises his system to contain 57 elements (Newlands had only used 24), and arranges them in order of increasing atomic weight. Definite groups and subgroups aand gaps in the series (which may have been predictions of undiscovered elements) were the main features of this system. However, he could not explain the relationships his table showed, assuming it would have to include valence.

1867
Gustavus Hinrichs, Iowa State, stated that the properties of of chemical elements are functions of thier weights. He proposed a spiral chart and also had vacant spaces.

1868
As early as 1864, Julius Lothar Meyer, University of Tubingen, presented a system based on valence, but he could not include all known elements. Eventually he did include them all, but he was not published until 1872, after Mendeleev. He is acknowledged as an independent discovere of the periodic law with Mendeleev. He never doubted the indivisibility of the atom, and showed that properties were functions of atomic weight, such as atomic volume.

1869
Dmitri Mendeleev uses the advantage of recent developments--precise atomic weights (Cannizzaro) and the large sample of known elements (67) to create a system of classifciation based on atomic weight. He was aware of these predecessors: Kremers, Gladstone, Lenssen, Dumas, and Pettenkofer. He knew nothing of the more important work of Newlands, Odling, Hinrichs, and Meyer at the time he formulated his table. Not only did he use relationships between elements with similar properties, but he also discovered relationships between elements with dissimilar properties. Whether or not Mendeleev used cards to sort his elements (his affinity for the card game patience is documented), he wrote a table which recognized bidirectional relationships, both horizontally and vertically. His predictions of the physical and chemical properties of undiscovered elements provided the experimental verification that made his theory accepted in just a few years. The discovery of gallium, germainiu, and scandium brought acceptance where a purely theoretical base for the table could not.

Bibliography

Primary Sources Secondary Sources

Representations of the Element Classification Schemes

Mendeleev's classification scheme is the best-known and is often presented to students as the best, if not the only classification of elements based on periodicity. In my classes, it is eventually used as a basis for extending the periodic law to explain valence and to introduce quantum theory. Mazur's book details over 100 variations of periodic tables. Different tables bring out unique relationships between elements, while disguising others. Students are asked to identify and explain an alternate classification system.

Activity 1: Elements and Conclusions

Groups: no more than four students per group, each student chooses a task as follows: Trays: each tray should contain a magnet and a conductivity tester (Flinn Scientific, 9 volt battery and LED for light bulb)

Elements: samples in open culture plates, with the exception of nitrogen

Properties: place one card in each tray

Teaching strategy

The Fable of the Lost Child

Like all good chemistry stories, this one begins ... Once upon a time, a child was visiting a landfill with his family. They were searching for used materials to recycle. Somehow, in filling their van with trash, they overlooked the fact that they had left one child behind. When the child discoverd he was alone, and that night was approaching, he realized he would need a fire to keep warm. So he went in search of fuel for a fire. He gathered these materials and discovered that some things would burn, and others would not. To avoid collecting things that wouldn't burn, the child made a list. This list was very helpful, but soon, flagpoles and tree limbs and broom handles became scarce. The child looked for a pattern in the list tha might lead him to new fuels. The next day the child went looking without his list, but he remembered his conclusion. He used it to bring back these materials-- successful predicitons. And he avoided others. The child became confident of his conclusions (and wondered where his parents were!), and returned to the fire with three pieces of pipe, two pop bottles, and the axle from an old car. He did not bring back a huge box of newspapers. During the long, cold night that followed, the child devised another conclusion. What would be a more useful conclusion?

[This story is paraphrased from the CHEMStudy text, Chemistry: Experiments and Principles.]

Lessons from the story can be organized using the following lists on the board or overhead:

Things That Burn

Things That Don't Burn CONCLUSION: Cylindrical objects burn.

- - - - - - - -
Cylindrical

Not cylindrical NEW CONCLUSION?...

Link to continuation of activities on Mendeleev.