Mendeleev’s periodic table,

For nearly half a century, Mendeleev s periodic table remained an empirical compilation of the relationship of the elements. Only after the first atomic model was developed by the physicists of the early twentieth century, which took form in Bohr s model, was it possible to reconcile the involved general concepts with the specificity of the chemical elements. Bohr indeed expanded Rutherford s model of the atom, which tried to connect the chemical specificity of the elements grouped in Mendeleev s table with the behavior of electrons spinning around the nucleus. Bohr hit upon the idea that Mendeleev s periodicity could... 

Gallium [7440-55-3] atomic number 31, was discovered through a study of its spectral properties in 1875 by P. E. Lecoq de Boisbaudran and named from Gallia in honor of its discoverer s homeland. The first element to be discovered after the pubHcation of Mendeleev s Periodic Table, its discovery constituted a confirmation of the Table which was reinforced shordy after by the discoveries of scandium and germanium. 

The nine elements, Fe, Ru, Os Co, Rh, Ir Ni, Pd and Pt, together formed Group VIII of Mendeleev s periodic table. They will be treated here, like the other transition elements, in vertical triads, but because of the marked horizontal similarities it is not uncommon for Fe, Co and Ni to be distinguished from the other six elements (known collectively as the platinum metals) and the two sets of elements considered separately. 

E. Barnes, On Mendeleev s Predictions Comment on Scerri and Worrall, Studies in History and Philosophy of Science, Part A, 36 801-812, 2005 S. Schindler, Use-novel predictions and Mendeleev s Periodic Table Response to Scerri and Worrall, Studies in History and Philosophy of Science, Part A, 39 265-269, 2008 D. Harker, On the Predilections for Predictions, British Journal for the Philosophy of Science, 59 429—453, 2008 E. R. Scerri, Response to Barnes Critique of Scerri and Worrall, Studies in History and Philosophy of Science, 36 813-816, 2005. 

Fig. L Fragment of Mendeleev s periodic table of 1904 showing the positions ofpredicted...

The second conceptual tool was Dmitri Mendeleev s periodic table, which listed the known elements in order of increasing atomic mass. The resulting organizational chart arranged elements so that those with similar chemical properties were grouped together in the same column. 

As you know, Dalton s atomic theory no longer applies in its original form, and Mendeleev s periodic table has undergone many changes. For example, scientists later discovered that atoms are not the most basic unit of matter because they are divisible. As well, the modern periodic table lists the elements in order of their atomic number, not their atomic mass. Of course, it also includes elements that had not been discovered in Mendeleev s time. Even so, in modified form, both of these inventions are still studied and used today in every chemistry course around the world. 

Newland s law of octaves chem An arrangement of the elements that predated Mendeleev s periodic table Newland s arrangement was a grouping of the elements in increasing atomic weights (starting with lithium) In horizontal rows of eight elements, with each new row directly beneath the previous one. nti-lonz 16 3v ak-tivz ... 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Niels Bohr was a physicist, not a chemist. I devote a chapter to his life because he was the scientist who explained why Mendeleev s periodic table had the properties it did. Widely known as a soccer player in his youth, Bohr became the most influential physicist of the first half of the twentieth century. His life, too, was touched by political events. A Jew living in occupied Denmark, Bohr had to flee the country to avoid arrest by the Nazis. In 1939 Bohr discovered a theory that explained nuclear fission, and suggested that uranium 235 could be used to make a bomb. Though he played only a minor role in the American atomic bomb project, Bohr was the first to ponder the political implications of the bomb. 

Mendeleev s periodic table (Figure 6.3) was used with little modification until well into the twentieth century. The top of each... 

Transfer the bowl into a muffle furnace and roast the precipitate at 400-450 °C during one hour. Cool the bowl with the substance to room temperature in a desiccator and weigh it. Use the found value of the equivalent and the approximate value of the atomic mass of lead to determine its valence. Calculate a more accurate value of the atomic mass of lead from the obtained data, compare your result with the value of the atomic mass of this element given in Mendeleev s periodic table, and establish the magnitude of the error in per cent. 

Indicate the position of oxygen in Mendeleev s periodic table of the elements, the size of its atom, the electron configurations of the atom and molecule, and its oxidation states. How can the paramagnetic properties of liquid oxygen be explained ... 

Indicate the position of the halogens in Mendeleev s periodic table, their electron configurations, the size of their atoms, and the exhibited oxidation states. Write the electron configurations of the halogen molecules. How does the binding energy change in them ... 

Give a general characteristic of the elements of Group two of Mendeleev s periodic table of the elements. Underline the similarity and difference in their properties, in particular their ability to form complex compounds. 

Write the electron configurations of tin and lead atoms. Give examples of compounds in which tin and lead exhibit various oxidation states. Give a general and comparative characteristic of Group four elements of Mendeleev s periodic table. 

That the nobility of Man, acquired in a hundred centuries of trial and error, lay in making himself the conqueror of matter, and that I had enrolled in chemistry because I wanted to remain faithful to this nobility. That conquering matter is to understand it, and understanding matter is necessary to understanding the universe and ourselves and that therefore Mendeleev s Periodic Table, which just during those weeks we were laboriously learning to unravel, was poetry, loftier and more solemn than all the poetry we had swallowed down in liceo and come to think of it, it even rhymed ... 

The great success of Bohr s model of Mendeleev s periodic table of the elements and the appUcabiUty of the Ritz formula for the energy levels show that treating the electron in an atom as if it were in a Coulomb field is a reasonable approximation. 

FIGURE 5.2 A portion of Mendeleev s periodic table, giving the atomic masses known at the time and showing some of the "holes" representing unknown elements. There is an unknown element (which turned out to be gallium, Ga) beneath aluminum (Al) and another one (which turned out to be germanium, Ge) beneath silicon (Si). 

The success of these and other predictions convinced chemists of the usefulness of Mendeleev s periodic table and led to its wide acceptance. Even Mendeleev made some mistakes, though. He was completely unaware of the existence of the group 8A elements—He, Ne, Ar, Kr, Xe, and Rn—because none were known at the time. All are colorless, odorless gases with little or no chemical reactivity, and none were discovered until 1894, when argon was first isolated. 

As a scientific idea, Mendeleev s periodic table was tested by making predictions about elements that were unknown at that time but could possibly fill the gaps. Three of these gaps are shown by the symbols and J in Figure 9.2. As new elements were discovered, they were found to fit easily into the classification. For example, Mendeleev predicted the properties of the missing element eka-silicon (f). 

Figure 9.2 Mendeleev s periodic table. He left gaps for undiscovered elements.

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