ie-Physics

in development

Experiment V-5

invention of the period chart for chemical elements

As discussed in Experiment V-3 and Experiment V-4, the concept of atoms had been proposed to explain constant combining ratios, but molecular formulas and atomic weights remained a matter of controversy.  And lacking any direct evidence, even the reality of atoms was doubted by many.  While the combining weight ratios could be calculated from careful measurements, the atomic weights could only be obtained from correct molecular formulas, and those were often just guesses.  Despite a number of tricks and rules to try to understand the large accumulation of chemical measurements, exceptions and a multitude of other difficulties left much confusion.  For example organic chemists found many isomers (compounds which shared the same combining ratio, but had different molecular weights). Friedrich Kekulé pointed out in 1860 that 19 distinct formulae had been proposed for the single substance, acetic acid (the sour taste in vinegar)!  Chemists badly needed a way to determine which was right.

To find a solution to the confusion Kekulé suggested, and a friend, Carl Weltzien organized and convened on September 3, 1860 the first International Chemical Congress at Weltzien's home institution at Karlsrude along the Rhine.  140 prominent chemists met to discuss their different views on atoms, molecules, equivalents, and rational nomenclature.  The Congress did decide to base atomic weights on Oxygen (=16 exactly) rather than Dalton's original preference for Hydrogen (=1) because Oxygen reacts with more elements than Hydrogen.  This reduced cumulative experimental errors in atomic weights.  But discussions at the conference about how correct formulae could be determined did not lead to agreement.

One of the participants, Stanislao Cannizzaro (1826 to 1910 photo at left) from the University of Genoa, had argued for atomic weights based on two assumptions by another Italian, Avogadro.  But Avogadro's hypothesis, first made in 1813, had been almost immediately rejected because it requires some elements to have molecules with identical atoms.  But if molecules are chemically bond together by electrical forces, electrically identical atoms must repel, not attract.  And furthermore, Avogadro assumed that all molecules, despite their size and mass, occupy the same amount of volume so that equal volumes of two different gases at the same temperature and pressure would contain identical numbers of molecules.  Few thought that even remotely reasonable.  The Congress disbanded with no agreement on how to determine atomic weights.  But a friend of Cannizzaro distributed at the close of the meeting reprints of a class syllabus by Cannizzaro.

I also received a copy which I put in my pocket to read on the way home.  Once arrived there I read it again repeatedly and was astonished at the clearness with which the little book illuminated the most important points of controversy.  The scales seemed to fall from my eyes.  Doubts disappeared and a feeling of quiet certainty took their place.

Cannizzaro had carefully found ways around the previous inconsistencies and exceptions.  But more importantly, he was able to show that Avogadro's hypothesis, no matter how unreasonable it seemed on the surface, actually provided a way to understand all the accumulated measurements and led to a logically consistent set of atomic and molecular weights.

Once atomic weights were finally determined correctly, patterns began to appear.  In 1864 Meyer (shown at left) published Die Moderne Theorien der Chemie... which established a wide reputation.  In Die Moderne Theorien der Chemie... Meyer adopted and popularized the Avogadro Hypothesis.  His text contained three tables with elements grouped in columns by their valence.  The tables do not appear in the section on classification of elements, but rather in a section entitled The Nature of Atoms: Evidence Opposing their Simplicity.  At a time when much of the world was still debating whether atoms really exist as the smallest entities of matter, Meyer pondered the possible nature of atoms.  His table with vertical columns of families of similar valence elements had increasing atomic weight elements in horizontal rows.  For the empty position between Silicon and Tin, Meyer applied half the weight difference above and half below but didn't postulate a missing element.  Meyer was struck by the periodicities of physical properties.

In 1868 Meyer went to Carlsruhe Polytechnicum which during the war between Germany and France was used by the army as a hospital.  He applied his medical training as an army surgeon and was awarded a medal at the close of the war.

In 1869 a German abstract of a March publication revealed that a Russian chemist, Dmitri Ivanovich Mendeléeff, (1834-1907, photo to right) has published a somewhat similar periodic table.  The abstract shows the table accompanied by eight conclusions from the end of the Russian paper.  Mendeléeff too attended the first International Chemical Congress in September 1860 at Karlsruhe and had been impressed by the paper on determining atomic weights Stanislao Cannizzaro distributed.  Mendeléeff immediately wrote his teacher in Petrograd (which was publish in a Petrograd newspaper and a Moscow journal) noting that Cannizzaro corrected inconsistencies in existing atomic weights of metals so that atomic heats divided by a substance's number of atoms resulted in a constant (about 6 to 7) in accord with Dulong and Petit.  Before returning to Russia he visited the new oil fields in Pennsylvania.  Upon his return to Petrograd in early 1861, he was granted his doctorate and appointed professor of chemistry at the Technological Institute.

In the Fall of 1867 he became professor of general chemistry at the University of Petrograd.  Not finding any suitable Russian textbook to recommend to students, he began to write The Principles of Chemistry, the first portion which was published in May or June 1868.  Mendeléeff searched for an organizing principle so that the mass of chemical knowledge would not be chaotic.  While the law of definite chemical compounds had persuasively proven the atomic theory, a whole group of so-called indefinite compounds and solutions seemed to refute the atomic theory.  Distinct chemical elements with well defined atomic weights clearly existed, but the actual existence and nature of atoms was more problematic.  On February 17, 1869 he completed his first periodic table which in March he presented in a paper to the Russian Chemical Society on The Relation of the Properties to the Atomic Weights of the Elements.  He fell ill and N.A.Menshutkin read it for him.  In this paper he presented a periodic table, boldly noted the periodic relationship of chemical formulas to atomic weights, noted some elements had atomic weights that needed correction, and forecast that new elements would be discovered to fit vacant spaces in his table.  The same month Mendeléeff published the second volume of The Principles of Chemistry, Chapters 12-22.  Paradoxically, it appears Mendeléeff found the weight of elements was an invariable characteristic that provided an organization for studying chemistry free from employing the notion of atoms that was shrouded in controversy.

The paper included eight conclusions including:

The properties of the elements show a periodic change with increasing atomic weight.

Most common elements in nature have low atomic weights.

The three elements (those right of aluminum and silicon, and below calcium) are required but not yet discovered.

Several atomic weights appear to be in error and need correction.

At the end of March 1970 the third volume was published and the final two volumes were published in February 1871.  In July 1871 Mendeléeff published a comprehensive paper on the periodic law.  But it was in February 1869 that Mendeléeff first realized that atomic weights, not valence should be the guiding principle for chemistry.  In subsequent editions, eight in all, he significantly revised the structure of the textbook to be in more accord with the periodic law with each new edition.

In December 1969 Meyer arranged 56 elements in a valence table that was published in 1870 with a series of graphs showing the periodic repetition of atomic volumes and other properties verses atomic weights.

In 1871 Mendeléeff produced a form of the periodic chart horizontal like Meyers, predicted atomic weights for nine missing elements, and gave detailed predictions of the properties for eka-Boron, eka-Aluminum, and eka-Silicon.

In 1871 Mendeléeff left blanks for 32 missing elements.

Unlike the traditional elements which provided a wealth of properties to organize, only a few of the rare earths were known making interpolation of their number and properties difficult.  Besides eka Cs and eka Nb above, there were a number of the 32 predicted elements that Mendeléeff predicted in error.  In 1889 Mendeléeff predicted details for dwi Te which was discovered in 1898 and called Polonium by the Curies.

Despite Mendeléeff's believe that some atomic weights were in error, more careful measurements of Ar (39.95) and K (39.10), Co (58.93) and Ni (58.69), and Te (127.6) and I (126.91) left them inverted.

The establishment of the periodic system was regarded by most chemists as a discovery of the highest importance.  Stanislao Cannizzaro had provided the key to establishing atomic weights with Avogadro's hypothesis.  A number of chemists had found patterns in those weights.  Mendeléeff had recognized the periodic table's broad power to organize all that was know about the chemical elements and their compounds plus the value for predicting yet undiscovered substances and their properties.  Meyers had first used the pattern to merely suggest the difficulty with simple atoms but quickly appreciated and popularized the broad significance suggested by Mendeléeff.  Alexandre Emile Béguyer de Chancourtois (1820 - 1886), professor of geology at the École des Mines in Paris, described to the French Academy of Sciences in 1882-3 his vis Tellurique (earthy screw) plotting atomic weights on a helix with 16 atomic mass units per turn.  (However Chancourtois' screw does NOT align any chemical families; he found no chemical periodicity.)  William Odling (1829 - 1921), a reader in chemistry at St. Bartholomew's Hospital in London had suggested an arithmetical seriation of atomic weights in 1864.  Gustavus Detlef Hinrichs (1836 - 1923), professor at the University of Iowa, published in 1867 a radial periodic chart with each family arranged along spokes with the monomer of all atoms, an Urstoff called pantogen with a weight of half hydrogen's at the center.  (But only non-metals had arcs of similar atomic weights.)  But it was Mendeléeff's broad appreciation for the significance and practical use of the periodic law for all elements that won him acknowledgement as its discoverer.

But the failure to immediately find the expected underlying unity of matter implied by the periodic table became frustrating to many chemists.  No Urstoff was found, yet there must be an explanation why the properties of elements reoccurred periodically.

In 1904, Mendeléeff extrapolated two elements lighter than Hydrogen, and several heavy elements.  Few of these predictions were accurate.

Experiment

to follow

Finally, record your procedures, measurements, and findings in your journal.  If you need course credit, use your observations recorded in your journal to construct a formal report

References

• Original papers at Classic Chemistry compiled by Carmen Giunta, Le Moyne College Department of Chemistry
• More original papers at Chem Team compiled by John L. Park, Diamond Bar High School, Diamond Bar, CA
• Aaron J. Ihde, The Development of Modern Chemistry, Dover, 1964, 1984.