What is an Element? by Doris Kolb Illinois Central College
Behold the heaven and the earth and all the elements; for of these are all things created. T h o m d Kempis
"What are elements?" a student is likely to he asked on his verv first chemistrv. ouiz. . Elements isoneof those words that has-a variety of meanings. I t can mean the n a t u i d elements that cause floods and hurricanes, or the heating elements in a toaster, or the consecrated elements of the Eucharist. Even in a strictly chemical context it has dual meaning. "The element- of chemistry" denotes basic chemical principles, hut "the chemical elements" are a groupof fundamental materials. The questiun ahove, of course, refers to the latter, that group of simple chemical substances from which all other material thines are .------. ~~-~ derived. The student ~robablvconsiders this question ridiculously simple. Many of his ancestors would have found it complicated. Earllest Elements There were philosophers in ancient Greece who believed that everything in the hiverse could be reduced to one simple elementary substance. Thales concluded that the fundamental suhstanre~waswater. Ananimenes helieved it wasair. Heraclitus thought thnt it was fire. Empedwlessugge:ested that there were four-basic elements: water, air, fire, and earth. T o these Aristotle added a f i f t h e t h e r , the element of the heavens. For centuries these were accepted as the elements that made up the universe. Much earlier the ancient Chinese had developed a yin-yang theory, which interpreted all reactions in terms of two elements, a negative yin and a positive yang. Later the Chinese recognized five chemical elements: wwd, metal, water, earth, and fire. The ancient Hindus also had aphilosophical theory of matter. Their theory included nine elements: water, earth, fire, air, ether, time, space, soul, and sense. Actually some of the substances we now class'@ as elements were known to the ancients, but they were not recognized as elements. We know that copper, lead, gold, and silver were known to the Egyptians before 3500 B.C. They also used bronze (an alloy of copper and tin) and tools made of iron, according to records in pyramids built around 3000 B.C. Mercury appears to have been discovered around 1500 B.C. The only non-metals known to the ancients were carbon and sulfur, or "brimstone" as it is called in Genesis. All of these elements either occur in nature in the free state or can be senarated from their ores a t fairlv low tem~eratures. 'l)uring the Middle Agesthe alc6emists spent much of their timesearching for the "philosopher'sstone," which they believed would change hase metals Into gold. Around 800 A.D. a n Arahian alchemist named J a h ~ su~gested r that all metals were varying mixtures of mercury and &ur. This theory does not seem quite so absurd when you remember that many metals occur in nature as sulfides and that heating these ores produces sulfurous fumes and often frees the molten metal. For hundreds of years the alchemists did much mixing and heating of mercury and sulfur in pursuit of the philosopher's stone. The 16th century is sometimes called the Iatrochemical Period. ~~.when concern about how to make medicines gained in importance. (latros is theGreek word for physician.) Paracelsus was prohahly the most famous chemist of this era. Still ~~~
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696 / J o m l of Chemical Education
under the influence of alchemv, he believed that all substances were composed of mercury, sulfur, and salt. He called these the three pr~ncipleaof matter, since he still considered the elements to tx fire, air, earth, and water. The ancient elements of the Greeks seem to have been gradually supplanted by the more concrete principles of Paracelsus, with salt taking the place of water and earth, mercury (because of its volatility) being substituted for air, and sulfur (because of its combustibility) replacing fire. By 1600the list of known metals included antimony, arsenic, bismuth, and zinc, in addition to the seven metals !mown to the ancients, and the non-metals carbon and sulfur. What Is An Element?
Our modem idea as to what constitutes an element was first expressed by Robert Boyle in his treatise "The Sceptical Chymist" published in 1661. He d e f i e d an element as a basic substance that could be combimed with other elements to form compounds but could not itself be broken down into any simpler substance. His definition is all the more remarkable because Boyle and his contemporaries still thought that gold could be made from other metals, and Boyle's list of elements included such materials as salt, water, and air. It was not until a century later that men actually began to identify which substances could he broken down into simpler substances and which could not. During the 18th century chemists became very interested in combustion. They explained the buming process in terms of a mysterious material called "phlogiston". When a log was burned, its apparent loss in weight was attributed to the release of phlogiston into the air Wood = Calr (ash) +Phlogiston f For substances such as tin, which gained weight upon burning,
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"What is An Element" is of I series of reviews of chkmical minciples taught fust in high xhwl chemistry courses. Hopehlly this series will provide a mechanism whereby secondary school teachers can recalibrate their thouehts and reaffum their mderstanding of some of the more lundamental concepts used repeatedly in their teaching. REVISITED appears in odd-numbered months. Dr. Kolb received a BS degree from the University of Louisville Doris Kolb Illinois Central College and both MS and PhD degrees EastPeoria,lllinois616~~ from The Ohio State University. She has~ -heen as a .-~ ~ ~ emoloved chemist at the Standard Oil Company and as a h e h i o n lecturer in a series "Spotlight on Research". She has sewed on the staffs of Coming Community College and Bradley University. Since 1967, she has been Professor of Chemistry at Illinois Central College. -
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they explained that negative phlogiston was being released. When Henw Cavendish in 1766 discovered a highly flammable gas that produced water, and Joseph ~ r i e s t i in k ~1774 discovered a gas in which thinas burned with exceptional vigor, both men explained'their discoveries in the light of the phlogiston theory, Priestley referring to his gas as "dephlogisticated air." Meanwhile the French chemist Antoine Lavoisier had become very concerned about quantitative relationshios in chemical reactions. He burned the flammable w s 2 ~ a v k d i s h(which he called hydrogen) in the d ( ~ p h l o g i ~ i catpd air of Priestlev (which he called oxveenl. and jhuwud that the weight of tge water produced was equal to the sum of the weights of the two eases. He intermeted the reaction as the simile combination u u
Hydrogen + Oxygen = Water From this he concluded that water was a comuound made UD of the elements hydrogen and oxygen. Lavoisier further ~ o i n t e dout that when a substance burns, the productsalways weigh more than the original s u t ~ s h n w , and that the increase in weight is due to the comhinntion of the oxygen in the air with the substance that is burned. Not only had he upset the phloaiston theory, but he had also laid do& a fundamental criterion for deciding whether substances were elements or compounds. In order for a substance to qualify as an element, its weight had to increase whenever i t was changed into a new substance. The first list of chemical elements that was at all similar to our modern list was published by Lavoisier in 1786. His list included 33 substances, most of which we still recognize as elements. He listed hydrogen, oxygen, nitrogen, sulfur, phosphorus, and carbon, plus seventeen metals (the eleven that were known before 1600 along with platinum, nickel, manganese, molybdenum, tellurium, and tungsten). Also on his list, however, were the "muriatic, fluoric, and boracic radicals" (from hydrochloric, hydrofluoric, and boric acids) and certain oxides (lime, magnesia, baria, silica, and alumina). Most surprising, in view of Lavoisier's interest in analytical weighing, is the fact that he included light and heat (caloric) in his list of elements. His "caloricn-the weightless material of fire-was in essence a kind of modified phlogiston. Humphry Davy disagreed with Lavoisier's caloric concept, maintainine that heat was not matter but a form of motion. Later ~ a also 4 decomposed by electrolysis some of the compounds Lavoisier had thought to be elements. By the early 1800's the question of which of the known substances were true elements and which were comnounds had been mainlv settled. Discovery of New Elements The 19th centurv was a orolific era of chemical discoverv. Now that the nat&e of elekents had been established, new elements were beine found with amazine"freouencv. Durine . the brief period from 1798 to 1804 chromium, nioGum, tantalum, palladium, rhodium, osmium, and iridium were all isolated. The electrolytic method perfected by Davy proved to be a valuable tool for determining whether very stable materials were elements or compounds. Using batteries of the type that had been recently constructed by Volta, Davy applied large electric currents to various com~ounds.Substances that would not yield to decomposition bybther methods could often be broken down by electrolysis. In decomposing these compounds to their component elements, Davy also isolated some extremely active metals. Within two years (1807-8) he had discovered potassium, sodium, barium, strontium, calcium, and magnesium. He also proved (1810) that chlorine was an element rather than a compound. Iodine and bromine were discovered not long after by others. The sodium and ~otassiumthat Davv had isolated were such powerful redu;ing agents that theibecame very useful for reducing other elements from their ores. For example,
boron was prepared from boric acid by reduction with potassium. Although J. J. Berzelius was primarily interested in the combining proportions of the elements and their atomic weights, he is also credited with the discovery of selenium, silicon, zirconium, titanium, and thorium, all obtained with the help of potassium as reducing agent. The aforementioned elements had all been discovered by 1830, along with a few others-lithium, cadmium, aluminum, and vanadium. (Aluminum was not isolated until 1825, even though it is the most abundant metal in the earth's crust.) The next few elements to be discovered were "rare earth" metals. Between 1839 and 1843 lanthanum, terhium, and erbium were separated from ores obtained from rich deposits of rare earth minerals located in the Scandinavian peninsula. The other rare earth metals were not discovered until later, lutetium being the last one to be found in nature in 1907. Ruthenium, the last member of the platinum group metals, was discovered in 1844. Then followed a 16-year period when no new elements were identified. Development of the Spectroscope The next important development, insofar as the discovery of new elements was concerned, was the invention of the spectroscope. The fact that certain substances can produce colored flames had been known since the 16th century. Robert Bunsen (the inventor of the burner) and Gustav Kirchhoff (a young physics professor) used a prism to split up the light from these colored flames. They found that all elements, if heated to incandescence, would produce characteristic patterns of color. In the spectroscope,which they built in 1859, an element was first introduced into a flame so that it gave off light. The light was allowed to pass through a narrow slit and then through a prism, so that it was separated into slit images of different colors. The characteristic set of spectral lines produced was called the spectrogram for that element. No two elements exhibited the same spectral pattern, and the wavelengths of the lines were not affected by the presence of other elements. The light intensities were also strong enough that trace amounts of elements could be detected. The spectroscope was a powerful new tool for elemental analysis. It was also the main reason why so many new elements were identified during the latter part of the 19th century. Almost immediately Bunsen and Kirchhoff announced the discovery of two new elements, cesium (1860) and rubidium (1861), named for their sky-blue and ruby-red flame colors. The spectroscope also enabled other investigators to identify thallium (1861) and indium (1863). Later the spectroscope would play an important role in the discovery of a number of other elements, especially the rare earths and the noble gases. Development of the Periodic Table By the mid 1860's the list of known elements had grown to 62. Some of the elements showed marked similarities in their properties. Johann Dobereiner in 1829 noted the existence of certain "triads." Chlorine, bromine, and iodine, for example, were very similar elements, with bromine lying halfway between the other two in color, reactivity, and atomic weight. The same kind of relationship existed for calcium, strontium, and barium; for lithium, sodium, and potassium; and for sulfur, selenium, and tellurium. Other chemists became interested in the mathematical relationship of the atomic weights of the elements. Various systems for classifying elements based on their atomic weights were proposed. Several suggestions involved the observation that the incremental difference in atomic weight for groups of similar elements was often 8, or some multiple of 8. An improved version of the atomic weight scale introduced by Cannizzaro in 1860 aroused new interest in the atomic weights of the elements. Graphs plotting atomic weight against various physical properties (e.g., melting point, boiling point, etc.) yielded remarkably similar curves showing periodic Volume 54, Number 11, November 1977 1 697
patterns of rise and fall. It was inevitable that someone would take an analogous look a t how chemical properties changed with increasing atomic weight. The first person to do this was a Frenchman named d e Chancourtois. In 1862 he listed the elements in increasing order of atomic weight in a helical arrangement along the surface of a cvlinder. He showed that a vertical line drawn down the sid'of the cylinder passed through elements with similar properties (such as lithium, sodium, and potassium). No one paid much attention. In 1866 an Englishman, John Newlands, listed the elements in order of increasing atomic weight so that they arranged themselves into eight vertical columns, producing seven horizontal families with similar properties. He saw a parallel between the musical octave and the repetition of chemical properties starting with every eighth eiement in the series. When he read his pauer describinc this "Law of Octaves" before the Chemical society, it w& received with mocking criticism. The editor of the Journal refused to publish it. Just three years later Dmitri Mendeleev, a Russian chemist, published a tahle classifying the elements in accordance with the neriodic reuetition of their nro~erties.His initial table was sim;lar to ~ewlands'iist:hut his hurizontal table (1871) was the forerunner of our mtrdern periodic tablr. In the Inttrr table rho elements were laid down horizontally ac. cording tu incrt.asin~atomic weight, w ~ elements h of similar propertirs being p1;wed one heluw the other. Seven vertical families of elements resulted, ranging from the most active metals in Group I to the most active nun-metals in ( h u p VII. There was also an eighth column to srrommodate elements in the iron and platinum groups, which did not seem to belong in any of the other seven groups. (A periodic tahle similar to that of Mendeleev wi~sdeveloped independently by n German chemist named Lothar Meyer.) Mendeleev left blank spaces whenever the elements did not seem to fit correctly. He also had enough confidence in his svstem to sueeest that the blank snaces indicated the existence o>elementsikat had not yet heen discovered. Three of these blank spaces lay directly below boron, aluminum, and silicon, and Mendeleev referred to the missing elements as eka-boron, eka-aluminum, and eka-silicon. He predicted the properties of these elements based solely on their locations in the table. Within fifteen vears all three of these elements (scandium. gallium, and g e k a n i u m ) had been discovered, and ~ e n d e : Irev's predictions prwed tu he astonishingly accurate. The periodic tahle l~roughtorder to the multiplicity of elements already known to exist, and it provided a framework onto which niw elements could he added as they were discovered. By 1886 eight more rare earth elements had been identified, and fluorine had finally been isolated as the free element. Around the close of the 19th century, a Scot, William Ramsay, found the inert gases helium, neon, argon, krypton, and xenon in extremely small concentration in the atmosphere. There were no empty spaces in Mendeleev's tahle for these new elements. hut i t was obvious from their atomic weights that they wduld fit easily into the periodic table as a new vertical column (Group 0). The last of the natural elements to be identified were hafnium and rhenium, which are so very rare in nature that they were not discovered until 1923 and 1925, respectively. ~
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Radioactive Elements Following the discovery of radioactivity by Henri Becquerel in 1896, Marie and Pierre Curie isolated two rare hut highly radioactive elements, polonium and radium. Two previously known elements, uranium and thorium, exhibited radioactivity but a t a much lower level. Within a few years actinium and radon were also discovered, and still another radioactive element, protactinium, was found in 1917. By 1925all the elements in nature had been discovered. The 698 1 Journal of Chemical Education
periodic table ended with element 92, uranium: however, there were still a few question marks remaining in the tahle in spots such as 13 and G I , and chemists cmtinued to look for these elrmrnts. It was n
