ie-Chemistry

Experiment A10

finding patterns to assist our understanding

Some human beings have found interest in finding both simplicity and patterns in our environment.  Alchemists have sought such in the materials of our world and often were able to hew out a living from that understanding.  Their name, as their craft, came partly from the Arab culture.  The precision of the Arabic language places al- to prefix a definite thing, the equivalent of the in the English language.  Much later, when Europeans developed new ideas and wanted to distance themselves from their Arab roots, they dropped the al-, becoming simply chemists.

At first materials such as water, air, soil and the fire seemed the most common, basic components of the material world.  By exposing to fire and often adding other secret ingredients, soil could be transformed into more valuable materials such as various metals.  As was the common practice, those could be sold or bartered by weight.  Over time even the secret recipes for transformations were often measured and reproduced by weight.

After many centuries, the insightful British and French chemists Robert Boyle and Antoine Lavoisier noted that when metals were formed from earthly substance, the metals were not heavier than their constituent earths, but lighter in weight.  They speculated that perhaps the metals were actually more elementary and that something was removed from the earth to release a metal.  That lead almost immediately to the proposal that the world's most elementary materials might be found by seeking materials such as metals which couldn't be made lighter by removing foreign ingredients.  Within a decade or so, approximately three dozen elements were identified and a two century-long search was started for additional new elements.

Within a quarter century of using balances as the empowering tool to monitor weight changes, it became accepted that some changes could be distinguished which became known as chemical reactions.  These were strange in that the ingredients only reacted in reproducible, set weight ratios, leaving any excess material unreacted.  Another brilliant Englishman, John Dalton, pointed out that this could be explained using an ancient idea that material only existed in tiny discrete lumps, called atoms.  The atoms attached together to form compounds in only pre-defined, simple integer ratios.  The discovery that a few elements could actually combine in several such ratios, but those ratios were themselves multiples (such as 1:1 and 2:1), seemed to confirm this atomic theory.  Materials formed by solution or simply mixing rather than chemical reaction could be combined in a range of concentrations, providing considerable confusion for many new to the atomic theory.

By comparing the weights of elements which combine with the element Hydrogen, it seemed obvious that each kind of atom must have a different amount of weight.  A buoyant material such as Hydrogen seemed to have light atoms while many dense materials such as Lead likely had heavier atoms.  But failure to find a a way to count the extremely tiny atoms stymied efforts to find an undisputed way to calculate their atomic weights.  Only after half a century did the Italian Stanislao Cannizzaro (b1826, d1910 ←photograph at left) point out that the solution was his fellow countryman Amadeo Avogadro's suggestion that equal volumes of different gases at the same temperature and pressure actually contain the same numbers of molecules.  Findings based on that could be used in turn to verify procedures used for materials not gases.  This immediately led the Russian Dmitri Mendeléeff (b1834, d1907, photograph at right→) to propose a periodic table of the elements, organized by increasing atomic weight in one dimension and combining ratio with Oxygen (which combines with more elements than Hydrogen) at right angles.  Elements with similar ratios and other properties are aligned next to each other in parallel columns, with elements composed of heavier atoms down and to the right on his 1869 and 1871 charts (reproduced below).

Note that in the time between presenting these two tables, Mendeléeff predicted the existence and properties of three missing elements in column 4 which were discovered in 1875, 1879, and 1886 and found to closely match detailed properties he predicted.

Experiment

In search for a way to organize the chemical elements to make the learning of chemistry easier, Mendeléeff wrote the chemical symbols and newly established atomic weights on a deck of cards, with each element on its own card.  He then arranged them, much as is done in a game of Solitaire.  After arranging the elements by atomic weight, he provided a second dimension of organization using his familiarity with the formulae of the compounds each formed.

You might recall that chemists as early as John Dalton tried to arrange atoms by atomic weights based on their combining ratios with Hydrogen.  But the first International Chemical Congress held in Karlsrude along the Rhine, September 1860, decided that since more elements combine with Oxygen, its compounds would be a better basis of atomic weights.  Mendeléeff was able to utilize his knowledge of such compounds as the basis for the periodic chart.  We can try to get an understanding of that creative process by using cards displaying the formulae of their known Hydrogen and Oxygen compounds.  With similar knowledge, it should be possible to follow his procedures and better understand how that discovery was made and its significance.

Procedure

1. Reproduce the table on paper.
   
   
2. Cut along the black lines to produce cards containing each element.  Note several spare cards are provided.
   
   
3. Arrange the elements along side each other in the order of the atomic weights, the numbers following the equal signs.  (Think of this as similar to arranging all the dates listed on a calendar in a single long row.)  Note Mendeléeff used the European custom of using a comma to represent a decimal point.  He added question marks where he wondered if the data he had might be in error.
   
   
4. Look for reoccurring patterns in the compound formulas.  Break the long arrangement, moving large segments to align elements with similar formulae along an unoccupied sides.  (Think of this like taking your long list of calendar dates, noticing some days such as Sundays have similarities, and moving whole weeks so that the similar Sundays are aligned as on a common calendar.)
   
   
5. Compare your periodic table with Mendeléeff's

Keep in mind the task was more difficult for Mendeléeff because he had to invent the procedure and risk his time and effort doing so when it wasn't clear that a useful organizational tool would result.

Reference

• Compounds listed in the table were obtained from WebElements, Mark Winter, The University of Sheffield, England