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The Periodic Table of the Elements
by Juan C. D√ľrsteler [message nļ 188]

When the Russian chemist Dmitri Mendeleev published the first version of his Periodic Table of the Elements in 1869 he couldn't imagine that it would become in due time one of the most outstanding information visualisations and that many fields would use it more than one century later as a visual metaphor.
Tabla_en.gif (41655 bytes)
Mendeleev's periodic table of chemical elements
: as can be seen at the Wikipedia   
Click on the image to enlarge it.

Mendeleev had no idea (we think) of Information Visualisation, but he definitely had a vast knowledge of Chemistry, his profession. The 19th century was very prolific in regards to the discovery of chemical elements and their associated properties. As different researchers were discovering new elements and the computation of atomic weight was being refined, many interesting coincidences were being discovered too. 

The work of different researchers had identified the fact that certain groups of elements had similar properties. Metals, for example, had a characteristic shininess, high conductivity both thermal and electrical and the tendency to combine with oxygen to produce oxides. The lapse between 1850 and 1865 was particularly fruitful in the discovery and characterisation of new elements. This gave rise to different taxonomies based on the similarity of their properties, like the triads of Döbereiner and Newland's octaves. So by 1869 there were already many pieces of the chemical elements puzzle.

Mendeleev had been studying their properties which he summarised in the following Periodic Law: 

The properties of the chemical elements aren't arbitrary, they depend on the structure of the atom and vary according to atomic weight in a periodic way.

Later on it was discovered that it was not the atomic weight but the atomic number* which defined the periodicity , but this subtlety is out of the scope of this discussion. The fact is that Mendeleev decided to follow the scheme of Newland's octaves, that ordered the elements into 8 columns according to increasing atomic weight, but correcting some of their limitations, among them: 

  1. Should an element not fit in the table according to its atomic weight, it was moved to another location for which the atomic weight was according to the periodicity of its properties, leaving a "hole" in the table for supposedly a yet to be discovered new element. 

  2. An in depth study was made regarding atomic weights that led to a higher accuracy in its determination and hence a better placement of the elements in the table. 

  3. Long periods were introduced for what today are known as transition metals, to avoid placing metals below non-metal elements.

From the use of the table we can derive three important features:

  • Knowledge Transmission. The table is very good at communicating in a very concise yet powerful way the existing knowledge about the properties of the elements.¬†

  • Pattern detection. The columns of the table correspond to groups of elements sharing the same chemical and physical properties. Today we know that all the elements in the same column have identical electronic configuration of the last layer, that is none other than the one that defines the valence and its interaction properties with other elements. Rows group elements of different properties but similar weights. Technically we would say that they have the same number of atomic orbitals. Something unknown in Mendeleev's time.¬†

  • Knowledge discovery. By leaving holes when no known element had the appropriate periodicity had the extraordinary effect of facilitating the discovery of new elements together with the prediction of its properties with great accuracy. Maybe this was one of the most outstanding features of the table in its time.

These three properties are typical of any good visualisation and have converted the periodic table in an example of excellence in information visualisation. 

Maybe due to this and to the fact that the table is very well known since it plays a part in many curricula of secondary schools worldwide,  you can find many attempts to use its layout as a way to visualise any type of classification, with greater or lesser success. 

Among them you can find the "Table of Condiments that Periodically Go Bad", listing condiments along with their nominal duration as consumable stuff, or the "Table of Desserts" that lists a set of desserts in a Periodic Table fashion. Much more recently appeared a Periodic table of Visualization Methods in visual-literacy.org due to the work of two researchers of the University of Lugano, in Switzerland. You can see the rationale behind it as a PDF document in "Towards A Periodic Table of Visualization Methods for Management".

TablaVisMet.gif (239124 bytes)
The periodic table of Visualisation Methods by Ralph Engler & Martin J. Eppler.
Source: as can be seen at Visual-Literacy.org. 
Click on the image to enlarge it.

Despite the good work in classifying more than a hundred different visualisation methods, using the scheme of the periodic table and the exact shape of the same for displaying the methods is more than disputable since the paradigm the periodic table adheres to (atomic number, chemical properties, orbitals, etc) has no parallelism to the case of visualisation methods, which invalidates the visual metaphor it intends to be. Stephen Few discusses this point very cleverly in his blog Visual Business Intelligence. Hence  I will not abound on this here.

The fact is that mimicking existing paradigms just because they provide a familiar lay-out doesn't add any insight into what we are looking for, that is regularities in the methods of visualisation. Trying to map the regularities of the chemical elements into those of desserts, or visualisations, is misleading since it hampers finding true regularities and although it covers the transmission of knowledge it doesn't contribute to pattern detection and even less to knowledge discovery, outstanding outcomes of Mendeleev work. 

Building a taxonomy of visualisation methods is not a simple issue and having an equivalent of the in depth work done by Mendeleev for chemistry in Information Visualisation would be a major advance, that, in my opinion we should pursue by finding the main features of each method, building a new paradigm and representing them in original and meaningful ways in accordance with said paradigm.

Incidentally a good reference for visualisation methods in information graphics is Harris' Information Graphics
Atomic number: Characteristic property of each element that represents the nuclear charge of the atom. It's equal to its number of protons.

Links of this issue:

http://en.wikipedia.org/wiki/Periodic_table   The periodic table at the Wikipedia
http://www.backtable.org/~blade/fnord/condiments.html   Periodic Table of Condiments
http://www.pugsly.com/10-FT4.jpg   Periodic Table of Desserts
http://www.visual-literacy.org/periodic_table/periodic_table.html   Periodic Table of Visualization Methods
http://www.visual-literacy.org/periodic_table/periodic_table.pdf   PDF article about the Periodic Table of Visualization Methods
http://www.perceptualedge.com/blog/?p=81   S. Few's discussion about the Periodic Table of Visualization Methods
http://www.infovis.net/printRec.php?rec=llibre&lang=2#InfoGraphicsRef   The book "Information Graphics: an Illustrated Reference" by R.L. Harris
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