Dmitri Ivanovitch Mendeleeff

While trying to organize a discussion of the properties of the elements for a chemistry course at the Technological Institute in Petrograd, Dmitri Ivanovitch Mendeleff listed the properties of each element on a different card. As he arranged these cards in different orders, he noticed that the properties of the elements repeat in a periodic fashion when the elements are listed more or less in order of increasing atomic weight.

In 1869 Mendeleff published the first of a series of papers outlining a periodic table of the elements. A copy of the table Mendeleff published in 1871 is shown in Figure 7.2. This table lists elements with similar chemical properties in the same column. All of the elements in the first column, for example, form compounds with the generic formula X₂O (such as H₂O, Li₂O, and Na₂O) while elements in the second group form compounds with the generic formula XO (such as BeO, MgO, and CaO).

A.N.: Insert Figure 7.2 here, if possible

Mendeleff was not the first to propose a periodic table in which the elements were arranged in order of increasing atomic weight. Several factors might explain why Mendeleff's contribution to the development of the periodic table is recognized as being so much greater than any of the other chemists who made similar observations.

1. Mendeleff grouped the elements on the basis of similarities in their chemical properties, not on an arbitrary mathematical rule such as the law of octaves. As a result, he didn't have to force the elements to fit a preconceived structure.

2. Mendeleff realized that additional elements would be discovered and left blank spaces at the appropriate places in his table where he predicted elements would occur.

3. Mendeleff questioned accepted values of atomic weights when they disagreed with the pattern of elements in his table. He concluded, for example, that the accepted values for the atomic weights of beryllium, indium, and uranium had to be wrong because they would result in these elements being placed in the wrong groups in the table.

4. Mendeleff accepted minor inversions in the order of increasing atomic weight when this placed elements in the correct groups. Thus, he placed tellurium before iodine, even though it had a slightly larger atomic weight.

5. Mendeleff predicted the properties of elements that had not yet been discovered, which allowed his theory to be tested.

Based on empty spaces in his periodic table, Mendeleff predicted the discovery of 10 elements, which he tentatively named by adding the prefix eka- to the name of the element immediately above each empty space. He then tried to predict in some detail the properties of four of these elements: eka-aluminum, eka-boron, eka-silicon and eka-tellurium.

The remarkable agreement between Mendeleff's predictions for eka-aluminum and the observed properties of the element gallium, which was discovered in 1875, is shown in Table 7.3. A similar comparison between the predicted properties of eka-silicon and the observed properties of germanium, discovered in 1886, is given in Table 7.4. It was the extraordinary success of Mendeleff's predictions that led chemists not only to accept the periodic table, but to recognize Mendeleff more than anyone else as the originator of the concept on which it was based.

TABLE 7.3

The Predicted Properties of Eka-Aluminum

Compared with the Observed Properties of Gallium

Mendeleff's Predictions Observed Properties

Atomic weight: about 68 Atomic weight: 69.72

Density: 5.9 Density: 5.94

Melting point: low Melting point: 30.15C

Formula of oxide: Ea₂O₃ Formula of oxide: Ga₂O₃

Formula of chloride: EaCl₃ Formula of chloride: GaCl₃

Chemistry: The hydroxide should Chemistry: The hydroxide dissolves in

dissolve in acids and bases; the in acids and bases; the metal forms

metal should form basic salts; basic salts; Ga₂S₃ is precipitated