Mendelev and the Periodic Table

May 17, 2012 — Leave a comment

The Periodic System:

“The triumph of modern science is generally regarded as resting on direct experimentation and the adoption of an empiricist outlook, which holds that only that which can be observed should count.” This is a good broad started point and to understand developments in science and the resultant philosophies of science one needs to understand the historical framework in which scientific discoveries happened. In the instance of chemistry and the elements Robert Boyle (1627 – 1691; widely considered the first modern chemist, coming from an alchemic tradition) and Antoine Lavoisier (1743 – 1794, named both oxygen and hydrogen and helped create the metric system) both took the view that an element should be defined by an appeal to empirical observations. They recommended an element should be defined as a material substance that has yet to be broken down into any more fundamental components by chemical means. These are the early developments in framing and structuring the periodic table. Many of the substances documented during htis period qualify as elements by modern standards, though some do not i.e. lumiere (light) and calorique (heat).

In the alchemic tradition dating back to antiquity, a handful of the elements were discovered and documented. From the mid-1700’s more of the natural elements started to be discovered through the innovations brought on by the Enlightenment and on into the 20th century where the discovery of radioactivity and nuclear fission made it possible to fill out the table.

Mendeleev:

Mendeleev conceived the modern periodic table during the time he was writing his textbook The Principles of Chemistry. His early work involved detailed examination of chemical properties as well as specific volumes of many substances.

“In lecture notes written between the years 1864 and 1865, for example, Mendeleev listed 53 elements but still continued to use the more outdated equivalent weights for 13 of them. By 1868, when he began writing the second volume of his textbook, he was listing 22 elements, all of them given their new atomic weights according to Cannizzaro. Whether this is a coincidence or not, it implies that by the time mendeleev had begun consciously to work on the classification of the elements, he had fully assimilated the use of the modern atomic weights, an approach that would prove to be so essential for his discovery.” (p104)

The concept of using atomic weight to place the elements in some sort of order is the heart of this development in chemistry. As an approach to ordering the elements, this idea had been around for long enough in the scientific community to call it pre-paradigm science (paradigm would be after the installation of the modern periodic table into the systematic study of chemistry). Five other discoverers of the periodic system, who had created their own individual systems are: De Chancourtois (the first to order elements in atomic weights), Odling, Newlands, Hinrichs, and Lothar Meyer.

Mendeleev’s great step was to order the elements in terms of increasing atomic weights, and compare them horizontally. Horizontal comparisons of groups of elements, ordered in increasing atomic weights.

“And so, in the space of a single day, February 17, 1869, Mendeleev not only began to make horizontal comparisons but also produced the first version of a full periodic table that included most of the known elements.” (106)

Mendeleev was able to make predictions using the periodic ordering system he had devised; most famously for the elements scandium, gallium, and germainium (all anticipated in this 1869 paper). He makes 8 key points with which he ends his 1869 publications:

1) The elements if arranged according to their atomic weights, exhibit an evident periodicity of properties.
2) Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (e.g. platinum, iridium, osmium), or which increase regularly (e.g. potassium, rubidium, caesium).
3) The arrangement of the elements, or groups of elements, in the order of their atomic weights corresponds to their so-called valences as well as, to some extent, to their distinctive chemical properties – as is apparent among other series – in that of lithium, beryllium, barium, carbon, nitrogen, oxygen and iron.
4) The elements which are most widely diffused have small atomic weights.
5) The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
6) We must expect the discovery of many yet unknown elements, for example, elements analogous to aluminium and silicon, whose atomic weight should be between 65 and 71.
7) The atomic weight of an element may sometimes be amended by a knowledge of those contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.
8) Certain characteristic properties of the elements can be foretold from their atomic weights.

The Aristotelian inheritance:

The long standing notion in chemical philosophy originating with Aristotle posits that chemicals (or the elements earth, fire, air and water for Aristotle) are to be considered abstract. The combination and proportion of these basic substances gave the characteristics of things in the world that could be observed, but the substances themselves remained abstract and unobservable.

Simple Substance:

Introduced by Lavoisier during the course of the chemical revolution in the 19th century. A substance that cannot be decomposed by any known means. Not all substances had to contain every one of these simple substances. There was no longer thought to be one undifferentiated primordial matter but instead a number of elementary constituents, or simple substances, now possessed of observable properties. A major departure from the Aristotelian tradition.

“For Mendeleev, the element was an entity, which was essentially unobservable but formed the inner essence of simple bodies. Whereas a particular ‘element’ was to be regarded as unchanging, its corresponding simple body aspect could take many forms, such as charcoal, diamond, and graphite, in the case of carbon. In this respect, Mendeleev may be thought of as upholding the ancient philosophical tradition regarding the nature of elements as bearers of properties. Mendeleev’s genius now lay in recognizing that just as it was the ‘element’ that survived intact in the course of compound formation, so atomic weight was the only quantity that survived in terms of measurable attributes. He therefore took the step of associating these two features together.” (p115)

Mendeleev took the idea of simple substance, and saw that an element must be the building block that makes a simple substance. Therefore, a simple substance takes many forms, but the element and its atomic weight stays the same.

Making Predictions

“Because he was attempting to classify abstract elements, not simple substances, Mendeleev was not misled by non-essential chemical properties.” (p118)

His focus on classifying the abstract elements allowed him to maintain the validity of the periodic law, even in instances where observational eveidence seemed to point against it. Mendeleev was able to distance himslef from the positivists philosophically by positing that there were three components of nature: matter, force, and spirit. No category is reducible to any other.

Reference: The Periodic Table: Its Story and Significance Eric R. Scerri Oxford Uni Press

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