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2019 International Year: Periodic Table of Chemical Elements

 

 

It has to be something big to convince the UN to commemorate it in an International Year. We get a glimpse of what convinced them from the press release in December 2017:

“The development of the Periodic Table of the Elements is one of the most significant achievements in science and a uniting scientific concept, with broad implications in Astronomy, Chemistry, Physics, Biology and other natural sciences …”

That is a lot of big words and big claims (and there are many more in the statement) about the Periodic Table, which is shown in full on the accompanying page. For beginners it must seem a big mystery as to why it is so important, and in this space we probably cannot solve that problem. But we can perhaps impart something of its essence.

For teachers and learners of science, it may have come as no surprise that the UN has recognised the Periodic Table, if only because it features quite strongly in the school science curricula — especially in GET Natural Sciences and FET Physical Sciences. In those curricula there is little time for history or reflection on significance, and there is a danger that one of the most significant achievements in science is not recognised as such.

Appreciating that may make the task of exam preparation more bearable. The Periodic Table now distributed by the Mail & Guardian was drawn up by the Wits School of Chemistry and is based on the latest knowledge of the known elements. There is a lot of information embodied in it and science teachers will immediately see this and understand much of that information.

The purpose of this supporting article is to describe the development of the Table in ways that help to bring out the achievement, rather than repeating the facts of what is there today. This scientific achievement rests upon thinking and experiment: hands-on, minds-on. Learning today is no different.

‘One of the most significant achievements in science’

There was no chemistry before the idea of a chemical element was properly established late in the 18th Century, although more than 2 000 years ago, philosophers were debating what the world is made of. They did not do experiments; they argued while reflecting on their environment. So many different materials, so many different behaviours of those materials.

One general idea was that we should assume there are a few simple substances (elements) out of which all other substances are made. A popular suggestion was that fire, earth, air and water are the primary ingredients of our world. Another was that all materials are made of atoms, and it was supposed these moved and formed bonds to each other. People talked and gave their opinions about the elements and atoms but could not “prove” anything.

This all changed when French scientist Antoine Lavoisier suggested we should study changes of materials and follow what is going on by mass measurements. He was the one who seriously used mass measurements to follow chemical changes — something that today is routine and a core idea in science curricula. Conservation of mass in chemical change was an experimental discovery he made.

Lavoisier then used this technique to develop the idea that elements could be discovered by heating them; substances that did not chemically change on heating, he said, are the chemical elements that all those philosophers had been talking about for centuries. Some elements cannot be broken down into anything simpler was the message when he published the first short list in 1789. And testing for breaking down meant you weigh — before and after heating!

With this breaking of the centuries-old logjam about how to decide what are elements, it became possible to classify pure substances that were not elements as compound substances (formed of course by combination of different elements). So when you learn about elements and compounds in grade eight, you are learning the results of Lavoisier’s experiments and hypotheses 230 years ago. This was the first vital step towards the development of the Periodic Table. Today, in whatever form of Periodic Table you find, you see the names of elements (six in the key) and a symbol (two in the key) for each one, as consequences of this breakthrough.

It did not take long (about 20 years) before the old idea of atoms was revisited. Dalton led this next big breakthrough (around 1808) and (following Lavoisier’s lead) proposed that elements are substances where all the atoms are of the same type, and in particular have the same mass. Different elements are made of different atoms. He made the first explicit link between elementary substances and atoms. As part of this he set about determining atomic weights (today also called relative atomic masses) by making a bold assumption — that atoms of different elements combine in a 1:1 ratio, unless proved otherwise.

Today, when we look at any Periodic Table we see relative atomic masses (three in the key), the more widely used name for what Dalton called atomic weights (still acceptable however) and we know, as he did, that they are relative — they have no units of mass, but instead compare the mass of one type of atom with a reference atom (H in Dalton’s time, 12 C today). Today, the masses of individual atoms are known: they are in the region of 10-24 g.

Luigi Galvani and Alessandro Volta discovered current electricity at around the same time, and this became another tool for the identifying of more elements. Electrolysis of water was first reported in 1801, an experiment that is popular today in schools around the world (in South Africa it appears in grade eight along with electrolysis of copper chloride solutions; the formation of elements by breaking down compounds is exactly what scientists were doing 200 years or so ago). An equally important result of discovering electrolysis was the idea that perhaps electrical forces held atoms together, something that gradually became more and more certain during the 1800s. It eventually led to the discovery of the electron as a charged sub-atomic particle, and later to the proposal of a nuclear model for atoms.

But we are forgetting about the Periodic Table, because long before Ernest Rutherford, Neils Bohr and later scientists had made their contributions to explaining the materials in our world, Dmitri Mendeleev marshalled the known facts of the time (1869) and drew a Periodic Table for the first time in a way that scientists found persuasive. He knew of far fewer elements than we do today, but he realised that when he arranged elements in a sequence of increasing atomic weight there appeared a regularity that seemed significant. He found that certain element properties repeated as the atomic weight sequence was followed.

Thus today we find a column of elements called alkali metals (group one) that have marked similarities, yet they have atomic weights that apparently are quite unrelated. Thus, following the increasing atomic weight sequence say, from lithium along the horizontal direction (called a period), it is only when sodium is reached that properties similar to lithium are found. Mendeleev was so convinced of this periodicity of properties (especially valency, the capacity to bond) that when he felt something was out of line he suggested there was a missing element, awaiting discovery. He was always right! The Table is rightly labelled as a Periodic Table.

Almost anybody can see that creating the first Periodic Table was clever, like completing a difficult crossword. The Table was loaded with significance, as it made the link between the “macro” world of substances and the “micro” world of atoms explicit, and pointed beyond confirming such a link to a whole deeper world of explanation. Periodic recurrence of characteristic element properties such as atomic size changes is not just a link but a pointer, to something that in his time was unknown.

Today we know what it points to. It points to the structure of atoms with their positive nuclei (charge reflected in the atomic number (one in the key)) and negative electrons around it organised in special sequences called electron configurations (five in the key). Knowing these things, scientists such as Gilbert Lewis proposed specific models of how atoms bond together, but only in accordance with rules — because valency varies with atoms, and some atoms do not bond. This is grade 11 stuff now, treated in very simple terms, but, even so, it’s very powerful for making sense of biology and some aspects of physics, as well as much of chemistry.

Today, new elements are still “discovered”, but not by decomposing compounds. In fact it would be more correct to say the new atoms are discovered by smashing nuclei together, and then seeing what results. Now a newly discovered atom is placed in the existing Periodic Table in accordance with its nuclear charge (atomic number) and nobody has really seen the element it represents. The element oganesson (at no 118) is placed in the group called noble gases, but it has never been seen; what has been seen (through instruments) are a few atoms of this element. Is the element a noble gas? Perhaps we have to follow Mendeleev and say: we believe in the periodicity, so it must be a noble gas.

Professor John Bradley is an Honorary Professor at the University of the Witwatersrand

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