The World of Atom (Part IV)

ReferenceA Logical Approach to Theoretical Physics



Chapter 19: Polyatomic Molecules – Stanislao Cannizzaro (1826 – 1910)

Elements combine as multiples of a certain quantity called their atomic weight. 

Chapter 20: The Periodic Table of the Elements – Dmitri Ivanovich Mendeleev (1834 – 1907)

The elements, arranged according to magnitude of atomic weight, show a periodic change of properties.

POSTULATE: The idea of atoms and molecules applies to chemical reactions.


Stanislao Cannizzaro (1826 – 1910) adopted a molecular, i.e., polyatomic, view of the elements, and showed that the atomic weights of elements, prepared in volatile compounds, could be deduced by the application of Avogadro’s hypothesis together with accurate combining weight data and vapor densities.

Cannizzaro’s greatest contribution was that “the different quantities of the same element contained in different molecules are all whole multiples of one and the same quantity, which always being entire, has the right to be called an atom.”

Dmitri Ivanovich Mendeleev (1834 – 1907) discovered that the elements, arranged according to magnitude of atomic weight, show a periodic change of properties. He arranged elements in vertical columns according to increasing atomic weight, so that the horizontal lines contain analogous elements, again according to increasing atomic weight. This resulted in a table from which several general conclusions could be drawn, such as, chemically analogous elements have atomic weights either in close agreement or increasing by equal amounts. The table showed new analogies, suggested corrections to some atomic weights, and predicted many new elements that were later discovered.

It showed that the elements most widely distributed in nature have small atomic weights, and all such elements are distinguished by their characteristic behavior. They are thus typical, and the lightest element, hydrogen, is therefore rightly chosen as the typical unit of mass. The magnitude of the atomic weight determines the properties of the element, whence, in the study of compounds, regard is to be paid not only to the number and properties of the elements and their mutual action, but to the atomic weights of the elements.

Mendeleev founded his system upon the quantity of the atomic weight because “the atomic weight is a quantity which does not refer to the momentary state of an element but belongs to a material part of it, a part which it has in common with the free element and with all its compounds.”

Thus, elements combine as multiples of a certain quantity called their atomic weight. This is similar to the later idea of “quantum”.  This “atomic weight” as quantum applies to chemical reactions. It refers to atomic configurations that are stable in themselves and in molecular combinations. The implication is that chemical combinations of elements in random quantities are not possible. It is this quantity or multiple of this quantity that exists freely in equilibrium with the background of primary substance also. The whirlpool model of the atom is consistent with this observation.


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