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p. 11. The elementslocked

  • Eric R. Scerri

Abstract

‘The elements’ outlines the history of the study of elements. Ancient Greeks had just four elements—earth, water, air, and fire—which they believed possessed the shape of the platonic solids. They also believed that the four elements each possessed abstract qualities, a belief which later evolved into the concept of periodicity. Some elements have been known for millennia. Others have required technological advancements in order to be isolated. Elements have been named by a variety of means, which has caused controversy among discoverers. Alchemists applied symbols to elements based on associated planets and celestial bodies. This was replaced by the lettered system we use today.

The ancient Greek philosophers recognized just four elements—earth, water, air, and fire—all of which survive in the astrological subdivision of the twelve signs of the zodiac. Some of those philosophers believed that these different elements consisted of microscopic components with differing shapes, and that this explained the various properties of the elements. The basic shapes of the four elements were thought to be those of the Platonic solids (Figure 1) made up entirely of the same two-dimensional shapes like triangles or squares. The Greeks believed that earth consisted of microscopic cubic particles. This association was made because, of all the Platonic solids, the cube possesses the faces with the largest surface area. The liquidity of water was explained by an appeal to the smoother shape possessed by the icosahedron, while fire was said to be painful to the touch because it consisted of the sharp particles in the form of tetrahedra. Air was thought to consist of octahedra since that was the only remaining Platonic solid. Sometime later, a fifth Platonic solid, the dodecahedron, was discovered by mathematicians, and this led Aristotle to propose that there might be a fifth element, or ‘quintessence’, which also became known as ether.

1. The Platonic solids, each associated with one of the ancient elements.

Today, the notion that elements are made up of Platonic solids is regarded as incorrect, but it was the origin of the fruitful p. 2notion that macroscopic properties of substances are governed by the structures of the microscopic components of which they are comprised. These ‘elements’ survived well into the Middle Ages and beyond, augmented with a few others discovered by the alchemists, the precursors of modern-day chemists. The best-known goal of the alchemists was to bring about the transmutation of elements. In particular, they attempted to change the base metal lead into the noble metal gold, whose colour, rarity, and chemical inertness has made it one of the most treasured substances since the dawn of civilization.

But in addition to being regarded as substances that could actually exist, Greek philosophers thought of the ‘elements’ as principles, or as tendencies and potentialities that gave rise to the observable properties of the elements. This rather subtle distinction between the abstract form of an element and its observable form has played an important role in the development of chemistry, although these days the subtler meaning is not very well understood even by professional chemists. The notion of an abstract element has nonetheless served as a fundamental guiding p. 3principle to some of the pioneers of the periodic system such as Dmitri Mendeleev, its major discoverer.

According to most textbook accounts, chemistry began properly only when it turned its back on ancient Greek wisdom and alchemy and on this seemingly mystical understanding of the nature of elements. The triumph of modern science is generally regarded as resting on direct experimentation, which holds that only what can be observed should count. Not surprisingly, the subtler and perhaps more fundamental sense of the concept of elements has generally been rejected. For example, Antoine Lavoisier took the view that an element should be defined by an appeal to empirical observation, thus relegating the role of abstract elements or elements as principles. Lavoisier held that an element should be defined as a material substance that has yet to be broken down into any more fundamental components. In 1789, Lavoisier published a list of thirty-three simple substances, or elements, according to this empirical criterion (Figure 2). The ancient elements of earth, water, air, and fire, which had by now been shown to consist of simpler substances, were correctly omitted from the list of elements.

2. Lavoisier’s list of elements as simple substances.

Many of the substances in Lavoisier’s list would qualify as elements by modern standards, while others like lumière (light) and calorique (heat) are no longer regarded as elements. Rapid advances in techniques of separation and characterization of chemical substances over the forthcoming years would help chemists expand this list. The important technique of spectroscopy, which measures the emission and absorption spectra of various kinds of radiation, would eventually yield a very accurate means by which each element could be identified through its ‘fingerprint’. Today, we recognize about ninety naturally occurring elements. Moreover, an additional twenty-five or so elements have been artificially synthesized.p. 4p. 5

The discovery of the elements

Some elements like iron, copper, gold, and silver have been known since the dawn of civilization. This reflects the fact that these elements can occur in uncombined form or are easy to separate from the minerals in which they occur.

Historians and archaeologists refer to certain epochs in human history as the Iron Age or the Bronze Age (bronze is an alloy of copper and tin). The alchemists added several more elements to the list, including sulphur, mercury, and phosphorus. In relatively modern times, the discovery of electricity enabled chemists to isolate many of the more reactive elements, which, unlike copper and iron, could not be obtained by heating their ores with charcoal (carbon). There have been a number of major episodes in the history of chemistry when half a dozen or so elements were discovered within a period of a few years. For example, the English chemist Humphry Davy made use of electricity, or more specifically the technique of electrolysis, to isolate about ten elements, including calcium, barium, magnesium, sodium, and chlorine.

Following the discovery of radioactivity and nuclear fission, yet more elements were discovered. The last seven elements to be isolated within the limits of the naturally occurring elements were protactinium, hafnium, rhenium, technetium, francium, astatine, and promethium, between the years 1917 and 1945. One of the last gaps to be filled was that corresponding to element 43, which became known as technetium from the Greek techne, meaning ‘artificial’. It was ‘manufactured’ in the course of radio-chemical reactions that would not have been feasible before the advent of nuclear physics. Nevertheless, it now appears that technetium does occur naturally on earth, although in minuscule amounts.p. 6

Names of the elements

Part of the appeal of the periodic table derives from the individual nature of the elements such as their colours or how they feel to the touch. Much interest also lies in their names. The chemist and concentration camp survivor Primo Levi wrote a much-acclaimed book called simply The Periodic Table in which each chapter is named after an element. The book is mostly about his relations and acquaintances, but each anecdote is motivated by Levi’s love of a particular element. The neurologist and author Oliver Sacks wrote a book called Uncle Tungsten in which he tells of his fascination with the elements, with chemistry, and in particular with the periodic table. More recently, two popular books on the elements have been written by Sam Kean and Hugh Aldersey-Williams. I think it is fair to say that the appeal of the elements in the public imagination has now truly arrived.

During the many centuries over which the elements have been discovered, many different approaches have been used to give them their names. Promethium, element 61, takes its name from Prometheus, the god who stole fire from heaven and gave it to human beings only to be punished for this act by Zeus. The connection of this tale to element 61 is in the heroic effort that was needed to isolate it, by analogy to the heroic and dangerous feat of Prometheus in Greek mythology. Promethium is one of the few elements that do not occur naturally on the earth. It was obtained as a decay product from the fission of another element, uranium.

Planets and other celestial bodies have also been used to name some elements. Helium is named after helios, the Greek name for the Sun. It was first observed in the spectrum of the Sun in 1868, and it was not until 1895 that it was first identified in terrestrial samples. Similarly, we have palladium after the asteroid Pallas, which in turn was named after Pallas, the Greek goddess of wisdom. The element cerium is named after Ceres, the first p. 7asteroid to be discovered, in the year 1801. Uranium is named after the planet Uranus, both the planet and the element having been discovered in the 1780s. In many of these cases too, the mythological theme persists. Uranus, for example, was the god of heaven in Greek mythology.

Many elements get their names from colours. The yellow-green gas chlorine comes from the Greek word khloros, which denotes the colour yellow-green. Caesium is named after the Latin cesium, which means grey-blue, because it has prominent grey-blue lines in its spectrum. The salts of the element rhodium often have a pink colour, and this explains why the name of the element was chosen from rhodon, the Greek for rose. The metal thallium gets its name from the Latin thallus, meaning green twig. It is an element that was discovered by the British chemist William Crookes from the prominent green line in its spectrum.

A large number of element names have come from the place where their discoverer lived, or wished to honour, such as americium, berkelium, californium, darmstadtium, europium, francium, germanium, hassium, polonium, gallium, hafnium (from Hafnia, the Latin name for Copenhagen), lutetium (from Lutetia, Latin for Paris), moscovium, nihonium (from Nihon or Japan), rhenium (from the region of the Rhine river), ruthenium (from Rus, Latin for an area which includes present-day western Russia, Ukraine, Belarus, and parts of Slovakia and Poland), and tennessine. Yet other element names are derived from geographical locations connected with the minerals in which they were found. This category includes the case of four elements named after the Swedish village of Ytterby, which lies close to Stockholm. Erbium, terbium, ytterbium, and yttrium were all found in ores located around this village, while a fifth element, holmium, was named after the Latin name for Stockholm.

In cases of more recently synthesized elements, their names come from those of the discoverer or a person whom the discoverers p. 8wished to honour. For example, we have bohrium, curium, einsteinium, fermium, flerovium, lawrencium, meintnerium, mendeleevium, nobelium, oganesson, roentgenium, rutherfordium, and seaborgium.

The naming of the later transuranium elements has featured nationalistic controversies and, in some cases, bitter disputes over who first synthesized the element and who should therefore be given the honour of selecting a name for it. In an attempt to resolve such disputes, the International Union of Pure and Applied Chemistry (IUPAC) decreed that the elements should be named impartially and systematically with the Latin numerals for the atomic number of the element in each case. Element 105, for example, was known as un-nil-pentium (Unp), while element 106 was un-nil-hexium (Unh). But more recently, after much deliberation on some of these later superheavy elements, IUPAC has returned the naming rights to the discoverers or synthesizers who were judged to have established priority in each case. Elements 105 and 106 are now called dubnium and seaborgium respectively.

The symbols that are used to depict each element in the periodic table also have a rich and interesting story. In alchemical times, the symbols for the elements often coincided with those of the planets from which they were named, or with which they were associated (Figure 3). The element mercury, for example, shared the same symbol as that of Mercury, the innermost planet in the solar system. Copper was associated with the planet Venus, and both the element and the planet shared the same symbol.

3. Names and symbols of ancient elements.

When John Dalton published his atomic theory in 1805, he retained several of the alchemical symbols for the elements. These were rather cumbersome, however, and did not lend themselves easily to reproduction in articles and books. The modern use of letter symbols was introduced by the Swedish chemist Jöns Jacob Berzelius in 1813.

p. 9A small minority of elements in the modern periodic table are represented by a single letter of the alphabet. These include hydrogen, carbon, oxygen, nitrogen, sulphur, and fluorine, which appear as H, C, O, N, S, and F. Most elements are depicted by two letters, the first of which is a capital letter and the second a lower-case letter. For example, we have Kr, Mg, Ne, Ba, Sc, for krypton, magnesium, neon, barium, and scandium, respectively. Some of the two-letter symbols are by no means intuitively obvious, such as Cu, Na, Fe, Pb, Hg, Ag, and Au, which are derived from the Latin names for the elements copper, sodium, iron, lead, mercury, silver, and gold. Tungsten is represented by a W after the German name for the element, which is wolfram.