Mendeleev's Periodic Table - Case Notes

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Author - Tom Huber – Summer 1998

Background The first 18 elements that Mendeleev worked with are the same as those given the students. Mendeleev organized the elements first by increasing atomic weight and then by chemical properties. Placing hydrogen aside for the moment, this results in two "complete" rows of 7 elements and 3 elements "left over" for the third row. The first two columns are unambiguous as the oxidation and reduction states match perfectly. Titanium is more difficult to place. Students will want to place it in the third column because it gives them a "complete" periodic table, but it really fits better in the fourth column, after which the prediction mentioned becomes obvious. We now know that atomic weight is correlated with atomic number, which is the true critical first criterion. The atomic number is the number of protons in the nucleus or the number of electrons in orbitals in the non-ionic form of the element. The way electrons are "packaged" gives rise to the periodicity reflected in this table. Without this periodicity we would just have one very long horizontal row! The so-called valence electrons (see any introductory chemistry text for details) determine the chemical properties of the elements and, hence, the columns in which the elements are located.

Teaching Notes Although many students will immediately have a notion of what to do with the cards (they had chemistry in high school), they will still have some difficulty with the task. This is because hydrogen is hard to categorize, the noble gases were unknown at the time, and the periodic table they will produce is not identical to the one they are used to studying. While trying to organize the cards, groups should be asked what criteria they are using. Using relative atomic mass (RAM) is unambiguous, and students will generally think of this early. However, discerning patterns using physical or chemical properties is more difficult, and students will often need help trying to make the best fit possible. Also, they must early on get over the "hydrogen problem." The other tricky part is where to place titanium, the last element they have. Many students will want to put it in the same column as boron and aluminum, thus having a "complete" periodic table, but it fits better in the next column, under carbon and silicon. After about 30 minutes of work, each group explains to the class their method of organization. A consensus (Ziman, ISS, p. 10) is determined, and each group writes a specific prediction. The prediction is the existence of an element between calcium and titanium in RAM, and with similar chemical and physical properties as boron and aluminum. Mendeleev predicted this element (scandium) in just this manner.

If a second set of cards is desired (for homework or instead of the elements) they are available in another word document. This set consists of straight-chain organic molecules with between one and four carbons, where the terminal carbon is in all possible oxidation states (e.g., n-alkanes, n-alcohols, n-aldehydes and n-carboxylic acids). There are 17 cards in the set because carbon dioxide is also included. Each card provides (1) the name of the molecule, (2) the molecular weight, (3) the physical state, (4) the standard heat of combustion, and (5) the boiling point. One card can be left out if desired (not carbon dioxide!). If students have just finished the elements exercise, they will immediately want to use molecular weight, which isn’t very useful here! The two most useful criteria are (1) number of carbons and (2) number of carbon-hydrogen (or carbon-oxygen) bonds in the last carbon. With this organization, specific patterns in heat of combustion (increasing from acids to aldehydes to alcohols to alkanes within a group of molecules all having the same number of carbons and increasing with the number of carbons within the same category) and boiling point (increasing with number of carbons within a group) will also become apparent, although the chemical understanding of them is beyond most freshman.

Reference List Any introductory chemistry book will provide ample background or one could consult Chapter 7 in Trefil, J. and R. M. Hazen. 1998. The Sciences; An Integrated Approach. John Wiley & Sons, New York, New York. ISBN 0-471-16117-9.