Text-book of medical and pharmaceutical chemistry / by Elias H. Bartley.

  • Bartley, Elias Hudson, 1849-1937.
Date:
1898
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of medical and pharmaceutical chemistry / by Elias H. Bartley. Source: Wellcome Collection.

    117/750
    The most successful attempt to find a natural system of classification of the elements is the one first proposed by Newlands and afterward developed by Mendelejeff, and is the only natural one in use. It is based upon the atomic weights, and is sometimes known as the periodic law. If a list of the elements be made, arranging them in the order of their atomic weights, from the lowest to the highest, the first seven (after hydrogen) will be found to be representative of seven groups of similar elements. (See table, p. 114.) Let each of these seven elements head a column, and arrange the rest under them in the order of their atomic weights, in lines from left to right. We notice that those elements resembling one another in their chemical proper- ties will be found together in the vertical columns. It will be noticed that the metals are near the bottom of the table, while the non-metals are at the top. In Mendelejeff's original table there are seven vertical columns, which are called the seven groups. There are twelve lateral rows, which are called series, or small periods. If these lateral rows or series be numbered, it will be noticed that the members of the alternate numbered series of a given period resemble one another more nearly than the adjoining numbered ones. Thus, in the first group: Li = 7, Kr=39, Rb — 85, Cs=i33, resemble one another more nearly than Na, Cu, Ag, Au. It will be noticed that hydrogen is arranged in the first period as the only one of that period. The table is imperfect in some cases, but in the main it brings together elements which form well-defined natural groups. As an example of such a group, take Nos. 2, 3, 5, and 7 of Group VII. With the increase of atomic weight they increase in specific gravity and consistency. Of the group, fluorine is strongly electronegative, while chlorine, bromine, and iodine grow less negative as the atomic weight increases. They all form acid hydrides containing one atom of basic hydrogen. They all possess a peculiar and somewhat similar pungent odor. They all have the leading valence of one, while the three last have a higher valence in certain oxygen compounds. While there are certain striking groups brought together by this arrangement, there are some irregularities. Thus, there is no series in which hydrogen finds a place. In making this element a period by itself, the last four members of the second period, C, N, O, and F, are made even-numbered, which removes them from the others of the same group which they most nearly resemble. Even with these imperfections the table is an attempt to classify the elements upon some natural basis, and has enabled its author to predict the discovery of at least three new elements,—viz., gallium, scandium, and germa- nium,—and describe their properties, years before they were discovered. This he was led to do from the vacant places in the table. 10