8- ANNALS OF HISTORY

THE : A WINDOW TO THE HISTORY OF ! SAVITA LADAGE & TEJAS JOSHI

The periodic table is central he periodic table is an integral part Early attempts to identify to , of the chemistry we study today. natural elements and it can be just as central TBut, have you ever wondered how in exploring the inspiring elements were discovered? Or, how the The belief that all in the world history and evolution of periodic table has evolved to its present around us is made up of a limited pool of chemistry as a subject. This structure and format – especially in the building blocks has persisted since ancient article embarks on this absence of advanced analytical techniques, times. This has led to numerous attempts, historical journey, with the instruments or accessible literature? The from different civilisations, to identify these objective of showcasing its answers to these questions lie in the building blocks. value for both teachers and unflinching human quest for knowledge, One such attempt identified four students. a logical approach, and a great deal of elementary substances – Water, , and foresight. As lucid and organized as it . added one more element – may appear today, the periodic table is in ‘Aether’, the element of the heavens, to this fact a reflection of the challenging and list. These finite building blocks were put uphill evolution of the very subject of together in a preliminary, but convincing, chemistry. Thus, learning about its history ‘table’ – becoming one of the first efforts is as invaluable for a teacher as it is for a to classify elements. Even back then, these learner… elementary substances were used to make of natural phenomena (refer Fig1)!

Fig. 1. A preliminary table of natural elements. This very old precursor, by Aristotle, to the modern periodic table might have been a modest beginning, but it worked well in explaining natural phenomena. For example, the presence/absence of fire explained hot and cold respectively. The absence of water implied a solid. Thus observing ash and experiencing heat following the burning of wood, wood was believed to be made up of earth and fire. Credits: Tejas Joshi. License: CC-BY-NC

REDISCOVERING SCHOOL SCIENCE 37 This simple, but reasonably rational, However, the first written record unique means of decomposition. It was classification prevailed for many documenting the discovery of an successfully deployed by Sir Humphry centuries. However, beginning with element dates back to 1669. The Davy to isolate the extremely reactive the work of alchemists (ancestors of element it describes is , and in 1807 and modern ), and later as a result discovered from urine, a natural source other alkaline like , of progress in experimental sciences, the of phosphates. and subsequently. concept of chemical elements began The reducing ability of potassium, While it is possible that many more witnessing a considerable change. in turn, helped Jöns Jacob Berzelius elements were discovered in this period, discover , , and it is difficult for us to establish the Zirconium etc. What are chemical existence of this knowledge. Alchemists elements? of this period relied heavily on trial and The gradual increase in the number of The discovery of chemical elements error. Also, given the potential economic known elements was accompanied by dates back to prehistoric times, when benefits that their discoveries might the evolution of ideas regarding the humans observed () bring them, they tended to refrain from and in the early being left behind from the burning of disclosing their learning. This meant that nineteenth century. Both these aspects wood in forest fires. knowledge was more likely to remain became important stepping stones An awareness of as many as seven isolated, and its development did not in subsequent attempts to classify – , , , , , progress methodically. elements. The proposed and , and the non- by in the beginning of Sulphur (apart from Carbon) – can Experimental science and the nineteenth century is particularly be traced back to ancient times. This the new concept of the significant in this context. Dalton may have been because many of these element suggested that elements were made up elements occur naturally, either in of indivisible particles, called ‘’. The first significant shift in our ideas free (elemental) forms or in like His idea that all atoms of a particular sulphides and oxides, and are easily about elements came from the work element were identical – in terms decomposed by simple heating or of in the seventeenth of their mass, size and properties – heating in the presence of charcoal. century. Boyle defined an element as focussed attention on the important It is also possible that once they were being a substance that could not be concept of atomic mass. According to discovered, their utility or importance broken down into simpler constituents, Dalton, the exact value for the atomic to humans may have driven their and could combine with other elements mass of an element could be thought of further identification. However, we to form a mixture (today’s compound). as being the signature of that element. have little documentation of either – Extensive work by scientists like This idea led to the question: how do their discovery, or their recognition as , and we calculate the atomic mass of an elements. in the eighteenth element? Dalton displayed remarkable century demonstrated this concept foresight in calculating this value In this context, the history of Gold experimentally. is particularly significant. Due to its relative to another element whose mass attractive lustre, Gold (along with Silver) Cavendish discovered a flammable was known (i.e. as a reference became a symbol of wealth (ornaments) (produced by the reaction of and element to predict relative masses of and beauty, gradually assuming metal), christened Hydrogen, at around other elements). significance as a medium of exchange the same time that Priestley discovered and international trade. Consequently, a gas, christened , which many alchemists began to seek this supported burning. Lavoisier’s milestone ‘Philosopher’s Stone’ by attempting to synthesis of water using the two was convert other metals, like Iron, the first major blow to Aristotle’s choice into Gold. It was these attempts by of elements. Lavoisier also established alchemists from the Middle Ages that the in chemical led to the discovery of many other reactions and provided a basis for elements, like Antimony, and writing chemical reactions. Bismuth. It also led to the development of a variety of glassware as well as An invention that played a vital role the discovery of three major – in the discovery of new elements was sulphuric, hydrochloric and nitric, all of the construction of the Volta’s cell in which have been crucial for subsequent 1800. The Volta’s cell provided a steady experimental research. source of and, thereby, a

38 Box 1. Atoms or ? Periodicity in chemical Remarkably, at this point of time, the chemical formulae of compounds was not known, properties of elements nor was the concept of . However, the law of conservation of mass (Lavoisier) and Following the 1860 Congress, the the law of constant proportion (Proust) had already been established. The law of constant considerable number of known elements proportion by Proust states that irrespective of its source, a particular compound (say water) is made up of the same elements (hydrogen and oxygen) present in a constant (63) and clarifications about their mass ratio (1:8) throughout. Keeping hydrogen as a reference, and assuming the simplest atomic masses, valence etc. provided the formula of water to be HO, Dalton concluded the atomic mass of oxygen to be 8. appropriate reference points needed to Gay-Lussac was working with chemical reactions in gaseous phase, and suggested that organize this information. atoms need not be the smallest particles in an element to have independent existence. Gay-Lussac’s results were in conflict with Dalton’s postulate of the indivisibility of an was the first to identify atom. This conflict was finally resolved by Avogadro who proposed the idea of ‘molecules’. a certain ‘periodicity’ in the chemical properties of elements. Newlands The concept of atomic mass and its calculating them, based on Avogadro’s observed that when arranged in order determination was further developed hypothesis, was presented at the of their increasing atomic masses in the period between 1800 and 1860 Congress. This landmark gathering thus (as calculated by Cannizzaro), every by Gay-Lussac, , laid the foundation for serious and eighth element to appear in sequence Berzelius, Jean Stas and Stanislao concerted efforts to reflect on existing from a given starting element in his Cannizzaro. Berzelius changed the knowledge about elements and their arrangement was similar to each other reference element from Hydrogen to properties. (refer Figure 2). He called this peculiar Oxygen, broadening the canvas of property the ‘Law of Octaves’ given its chemical assays by making use of readily Box 3. Triads of elements. similarity to the musical octave. available oxides. This historic notion of Even though most attempts at The fact that Newlands relied more on using some reference for calculating classifying elements happened after atomic masses of elements, rather than atomic masses is still very much in use – the Congress, there were with the 12C being the standard some noteworthy attempts prior to it, their physical and chemical properties, today. notably Dobereiner’s work. Dobereiner’s resulted in some limitations in his categorization of elements was arrangement. This was partly because Box 2. Developing the idea of based on their chemical similarity: some of the atomic mass values that atomic weights. he arranged a group of three similar were in use at that time were incorrect, elements in an increasing order of (their The development of the idea of atomic then known) atomic masses. When this leading to their incorrect placement. weights is an amazing story in itself, was done, he noticed that the atomic Additionally, Newland did not leave gaps which has been discussed in detail in for undiscovered elements in his table. an article titled – The Saga of Atomic mass of the middle element was close Weights (Pg. 78) in the first issue (Nov to the average of the mass of the other 2015) of i wonder… We’d recommend a two. He published his ‘Law of Triads’ quick read! in 1829, leading to the subsequent Mendeleev solves identification of ten such triads by the puzzle 1843. Thus, by the mid-nineteenth century, Even though Newlands identified almost 60 elements had been discovered, periodicity, his attempt at classifying and their atomic masses had been elements was not taken seriously by calculated. However, this knowledge chemists, and thus he did not pursue his was still largely unknown within the ideas further. It was , scientific community, and wasn’t who in 1869, and later in 1871, particularly accessible to everyone. As published his version of a periodic table. a result many conceptual ambiguities Fig. 2. Examples of some triads of This elegant system of classification regarding valence, molecular masses, elements. not only established the periodic law equivalent masses continued to prevail. Credits: Tejas Joshi. License: CC-BY-NC. convincingly; but, also anticipated some undiscovered elements by virtue of The necessity for contemporary chemists to come together to resolve However, this preliminary classification logical foresight, accommodating them could not be used to organize all known these ambiguities resulted in the first by actually leaving gaps! elements; neither was this grouping international Congress held at Karlsruhe, perfectly robust: for example, some What makes Mendeleev’s efforts, in Germany in 1860. Cannizzaro’s values tetrads, and even one pentad, were the development of the perioidic table, of atomic masses and his rationale for identified later! transformational?

REDISCOVERING SCHOOL SCIENCE 39 The first of these challenges came with the discovery of rare earths, i.e. lanthanides, using a spectroscope, invented by and Gustav Fig. 3. A portion of Newlands’ periodic table. Newland’s brave attempt in arranging elements Kirchoff in 1859. The spectroscope is and identifying periodicity is evident in this portion. (Cl), the eighth element to appear an instrument that allows us to detect in sequence from F (Fluorine) exhibits similar chemical properties to it. That they both belonged very small quantities of elements in any to the same group – of halogens – was established much later. substance, without chemically isolating Credits: Tejas Joshi. License: CC-BY-NC. them. For the rare earths, which were chemically very similar and difficult to separate, therefore, the spectroscope 1. Mendeleev’s arrangement did not predicted the presence of an element was apt. As many such rare earths were rely only on the atomic mass values that he called eka aluminium, and was being discovered post 1870, placing calculated by Cannizzaro. Instead, he later discovered and named ). pursued his own analysis of compounds, This visionary, and rather audacious, them in Mendeleev’s periodic table identifying chemically similar or provision in Mendeleev’s table allowed was proving to be a challenge. Their ‘analogous’ elements, and prioritised this more elements to be incorporated. In chemical similarity was at odds with chemical similarity in his arrangement. fact, it also guided the search for new the fact that one of the main attributes In fact, he used this information to elements! of Mendeleev’s classification was a question the atomic masses of several progression in chemical properties. Amazingly, a strikingly similar periodic elements. In 1905, successfully table was independently devised at resolved this problem by placing the rare 2. Mendeleev seemed to have taken around the same time by Lothar Meyer, earths between alkaline earth metals knowledge available to him as a a scientist who was later given almost and transition elements in his very long challenge –somewhat like a jigsaw equal credit for his contribution to the periodic table, comprising 33 columns! puzzle that was supposed to be development of the periodic table as assembled. He, therefore, prepared Mendeleev. Meyer’s table drew more Isn’t it remarkable that Werner was able individual cards for each element, attention towards a progression in to rightly place the rare earths without attempting to sort them through physical properties of elements, such as any knowledge of their electronic multiple arrangements. While their atomic volumes. configuration? considering these arrangements in Another challenge came with the the vertical and horizontal directions, A bigger problem: the discovery of the first inert gas – that we are familiar with (in today’s , in 1894, by table), he chose to use family likeness surge in new elements and Lord Rayleigh. This discovery vertically, and increasing atomic masses and accommodating them! was unwelcome to most chemists, as horizontally. Even as Mendeleev’s periodic table offered a formidable organization of the chemically unreactive Argon seemed 3. Mendeleev’s conviction enabled 60-odd elements known at the time, it to threaten all that we had discovered him to confidently question existing was soon threatened by the discovery of and understood about elements. The knowledge (for example, incorrect many new elements. subsequent discovery of other inert atomic masses) and he resorted to re- like , , and calculating or re-positioning apparently Xenon, amplified this complication – incorrectly placed elements. This culminating in the creation of a unique conviction and foresight were proved group in the table, placed between the correct subsequently. halogens and alkali metals. 4. Possibly the most salient feature of In 1898, and her husband, Mendeleev’s table are its blank spaces Pierre, discovered and , that were aimed at accommodating and by 1911, almost 30 radioactive some undiscovered elements. But these were not just spaces, they were elements were known. These discoveries also accompanied by predictions of presented yet another challenge to the the properties these yet undiscovered conceptual understanding of elements elements were likely to exhibit (for – mainly because some of them had example, having anticipated properties identical chemical properties, but similar to aluminium, Mendeleev different atomic masses. This naturally

40 Fig. 4. An overview of the discovery of the elements. Credits: Adapted by Tejas Joshi from Goldwhite, H., & Adams, R. C. (1970). Chronology of the discovery of elements. Journal of Chemical Education, 47(12), 808. led to the problem of how and where for the first time. Because of his work, creation of this trans- element to place these new elements in the atomic number (that is, number of in 1940 by Edwin McMillan and Philip periodic table. and present in the nucleus of the Abelson, at the Berkeley Radiation Kazimierz Fajans resolved this problem atom), instead of atomic mass, is now Laboratory was followed by extensive by suggesting that all (elements considered the signature of an element. syntheses of transuranium elements with identical chemical properties) of Moseley’s work also conclusively showed by Glenn Seaborg and his co-workers. an element must be placed along with that there were 14 rare earths, with the Accommodating these newly created it, in one single place, despite having two, till then missing, elements Hafnium elements in the periodic table was different atomic masses. and Rhenium being discovered soon yet another challenge, as nobody had after, by the X-ray method. anticipated them! By 1944, Seaborg, Atomic number: the new who christened this set of elements The latest addition to the periodic as the ‘’ group, had developed signature of an element table is that of new elements created an updated version of the table with Henry Moseley’s work in 1913 by humans. As a result, the concept of these elements placed below the rare showed that there exists a systematic elements evolved from being limited to earths (Lanthanides). This was based mathematical relationship between the naturally occurring ones to including on the discovery that the actinide placement number of an element in the those created in laboratories by the group of elements were analogous to periodic table and X-rays produced by transformation of matter on nuclear their corresponding lanthanides, and the element. Thus, he could measure bombardment. was the subsequently aided the identification of the atomic numbers of several elements first element to be synthesized. The many more synthetic elements.

REDISCOVERING SCHOOL SCIENCE 41 Fig. 5. Could the periodic table look different? Presented in this image is one example of a significantly unusual format – a spiral table developed by Theodor Benfey. With hydrogen placed at the centre of the spiral, the emergent spirals expand into eight segments, housing transition elements, lanthanides and . The spiral and helical models are not new – the Telluric Screw model proposed by Chancourtois in 1862 was a prominent example of a helical model. To explore other such formats, collected from across the world, visit Mark Leach’s online collection at http://www.meta-synthesis.com/ webbook/35_pt/pt_database.php. Credits: DePiep (Own work), Wikimedia Commons. URL: https://en.wikipedia.org/wiki/Alternative_periodic_tables#/ media/File:Elementspiral_(polyatomic).svg. License: CC-BY-SA.

Box 4. Resources An extensive list of print and web- based resources on the periodic table that we have referred to for this article, as well as some teaching resources developed by us, are openly accessible online at www. bit.ly/lmtce under the ‘Important References and Resources’ section on the portal. We recommend visiting the portal, which was built to make available educational resources for multiple audiences, some of whom might not be able to access teaching aids or international books in print. We are sure that some of these resources will inspire and support you in your practice – whether it is through designing activities for your students, directing them towards self-learning, or encouraging them to question and seek answers. We also offer print versions of a set of learning resources designed to act as lucid starting points for inculcating an appreciation for the periodic table and its elements among your students. These resources are available on purchase, and details for the same can be obtained by writing to us.

Fig. 6. Resources developed at the Homi Bhabha Centre for Science Education. Credits: Tejas Joshi. License: CC-BY-NC.

42 The periodic table as an These revisions could involve changes in progress is a constant human endeavour. technical information, or the addition of educational tool We present this historical journey in the new elements. The most recent version So here we are today, with the widely hope that it broadens your perspective of the table (January 2016), a standard on the periodic table (or anything that recognized long-form of the periodic reference for educators, incorporates you study in science for that matter). table. A lengthy journey, right? And four new elements that have been in the Rather than seeing it as a completed one that has not yet ended – efforts to news, and are known simply as 113, 115, product, we hope you can now see improve the functionality and format of 117, and 118. the periodic table continue (refer Figure the periodic table as the result of an 4 for one such example)! What makes the periodic table on-going, and rather captivating, story invaluable for chemistry, and science with characters who were curious, All revisions in the periodic table are education in general, is its extraordinary hard-working, and pursued questions documented and updated by a global depiction of the dynamic but gradual with no obvious answers through logical body called the International Union of process by which scientific knowledge contemplation. Pure and Applied Chemistry (IUPAC). progresses, and how pushing this

Tejas Joshi is pursuing a Science Education M.A. at the University College London Institute of Education. This follows a sustained stint at the Homi Bhabha Centre for Science Education since 2009, where Tejas first worked on a multi-component and later on creating visual learning resources for chemistry. His recent projects have been on context-based learning, laboratory chemistry education, open educational resource development for chemistry education and communication. In his free time, Tejas enjoys painting, illustrating and gardening. He can be contacted at [email protected].

Savita Ladage is a faculty member at the Homi Bhabha Centre for Science Education, TIFR, with a Ph.D. in chemistry education. She has been centrally involved in the chemistry Olympiad programme in India – designing questions, laboratory tasks and overviewing selection stages for more than 15 years, along with the NIUS chemistry programme for undergraduate education. Her academic interests include , chemistry education, particularly misconceptions, and developing experiments for undergraduate chemistry. She can be contacted at [email protected].

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