Question

Tutorial: History and Evolution of the Periodic Table of Elements
This tutorial will be divided into four parts: Pre-History; Alchemy through the Middle Ages; Early "Laws"; and Tables through the 20th Century.

Answer 1

Part I: Pre-History
The first elements to be widely recognized and used by human civilization are often referred to as the seven "ancient" metals. These metals are: copper, lead, gold, silver, iron, tin, and mercury. The simplest story about the discovery of these elements comes from the accidental “melting” of certain kinds of rocks in the cooking fires of pre-history. When certain kinds of green rocks were placed too near the fire, a brownish-colored substance seemed to pour out and, when solidified, could be pressed, hammered, flattened, and used as a better blade or sharp point than chipped rocks. This made acquiring tomorrow’s dinner significantly easier, as it meant less time had to be spent fashioning new spears every night when the fancy metal (copper) spearheads could be reused again and again. Copper has been discovered in artifacts dating to over 9000BC.

As a species, we humans tend to do a great job finding patterns and relationships between various aspects of our daily lives. It’s what helped us remember what to plant, when to plant, when to harvest, (where to poke the mammoth hardest so it dies quickly and we have dinner as quickly as possible). The downside is that we tend to do such a great job finding patterns that we begin to associate objects and events where no correlation is justified. After the campfire died down, and before hunkering down in the cave for the night, our ancestors looked up at the night sky, and noticed things: shiny, twinkly, moving things. After long enough, our ancestors started naming these things, seeing pictures in these “heavenly bodies”, telling stories about the people and animals and creatures that must inhabit such a wonderful place as “above”. Scattered among the stars, which seemed to retain their relative orientations, were seven “wanderers”. Today we know them as: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn. These wanderers moved differently than the stars, and so must possess some special significance.

“A-HA” cries our primitive ancestor-brain. “There must be some importance to that” and we begin to incorporate these celestial bodies into our personal narrative, making them into the personalities that would eventually become beings like the Egyptian, Greek, and Roman gods, believing their influence to extend from the celestial down to our terrestrial world (anyone who has ever read their own horoscope does the exact same thing today).

Each metal was seen as the Earthly embodiment of a particular god: Mars, red and warlike was given the element iron, for its usefulness in crafting weapons of war. Copper, mined heavily on the Mediterranean island of Cyprus, was given the personality of the goddess Venus, for whom the island was considered sacred. Jupiter was given tin; Mercury for mercury’s quick nature; Saturn lead; silver for the Moon; and the golden Sun given the purest metal of all, aurum, or gold. Each earthly element was seen as the blessing of its respective deity, possessing the properties and personalities of its maker.

Noblest of these heavenly gifts was gold. Naturally, perhaps, because of its color, or perhaps due to the resilience of pure gold. No single acid solution will dissolve a sample of pure gold. Metals and alloys of other compositions will dissolve in strong acid solutions, but pure gold will not. This is often said to be the rise of the statement “to pass the acid test”. A mixture of hydrochloric and nitric acids, called “aqua regia” (royal water) is needed to dissolve pure gold. The lowest, basest metal is lead: slow, dense, dull. Lead is the old man, waiting to die, personified by the god Saturn, or the Egyptian Osiris, god of the dead. Lead was also chosen as the symbol of the student: dull, dense, slow, uninitiated in the ways of knowledge.

The properties of two of the 7 ancient metals are also linked to the “properties” of the human elements: man, and woman. Iron, used for weapons, is given to the warlike Man, symbolized by the shield and spear. The vain lady holding her copper mirror to her face becomes Woman with all of her flaws, including turning green with envy, literally! The metals are seen as this heavenly gift, a kind of path to follow. If one is to transcend the mortal, base nature, and become pure and noble of spirit, the way MUST lie through the manipulations of these metals.

At the moment, still some 4000 years ago, all we’ve got are a small sample of metals, planets, and even the days of the week all linked together by some spiritual connection. There is no real need for a “table” of organization with such a small list of known elements. What this spiritual connection DID create, however, was the beginnings of what would turn into Chemistry as a legitimate science. The search for the purification of the metals became synonymous with the purification of the soul itself, and gave birth to the mystic art of Alchemy, the pre-science that would eventually become modern Chemistry.

Answer 2

Part II: Alchemy Through the Middle Ages (to the end of 1700)

As far back as 2000BC, we had a rudimentary working knowledge of our environment. We had extracted metals from their ores, baked mud into bricks, boiled fat with ashes to make soap, fermented grain into beer, and a handful of other simple chemical processes to make use of the resources the environment provided us (Gonick). What was not known, however, is the reason for these wonderful transformations. The Ancient Greeks had a host of ideas, the two most-popular coming from Aristotle and Democritus.

Democritus and his contemporaries proposed that, if one were to take a bar of silver and cut the bar into successive halves, eventually you would reach the point past which you cannot cut: an atom of silver. Other substances would have similar atomic limits. Aristotle presented a much simpler, and more visibly convincing kind of argument. “Consider,” Aristotle might have said, “the log on the hearth. Democritus would have us believe that the log is made of tree atoms, and by cutting and cutting and cutting, one could isolate the essential “tree-ness” of the tree. This is preposterous, as it would create a world filled with tree atoms, and rock atoms, and grass atoms, and marble atoms, and so on and so on…”

“In reality, the world is made of just four elements: Earth, Air, Fire, and Water. Other substances are just combinations of these four. The log on the hearth contains Fire, which we release when it burns. It contains Air, which we see rising as the smoke. It contains Earth, which is left behind once the other two have gone. It contains little to no Water, for when logs from the river are placed in the hearth, no Fire is released.”
Key in Aristotle’s theory was the desire of these elements to actively seek their natural states of rest. Rain falls because water doesn’t belong in the air. Rocks sink because earth doesn’t belong above water. Smoke rises, rocks roll downhill, and a host of others, easily seen in everyday experiences. Because his theory was based on direct observation rather than an imaginary cutting to some infinitely small size, Aristotle won the day over Democritus, and we started on a 2000-year long dead end. This led the ancient Greeks, Egyptians, and Arabians to seek the method of these transmutations. They sought to alter the recipe of natural things in order to create other things. The flame of the old quest to turn lead into gold was reborn, and the mystic art of alchemy (from Arabic al-kimia: from ashes) was set into full swing.

During this time we boiled, burned, baked, cooked, chopped, digested, fermented, decomposed, and otherwise mutilated our way through most natural substances we could get our hands on. Arabic, Egyptian, European, Chinese alchemists, just to name a few, literally invented the processes and laboratory terminology that we still use today. Famed Alchemist Jabir ibn Hayyan (Geber) is credited with the invention of over twenty types of laboratory equipment and processes. His diligent bookkeeping led to the repeatability of experiments by later alchemists and the beginnings of Chemistry as an experimental science. Geber’s contributions group him with Boyle and LaVoisier as one of the most important figures in the history of Chemistry.

German alchemist Hennig Brand took this search for gold literally: attempting to “isolate” metallic gold out of every substance that had the proper kind of yellow color, including urine! In 1669, Brand boiled over 5000L of urine until a bright red liquid dripped out and burst into flames in mid-air. By catching the liquid in water and allowing it to cool, Brand noticed that it formed a pale green-white solid that glowed with an eerie light. This light was constant, never diminished, and must have been an awesome sight to the Medieval mind. Brand named this discovery phosphorus, Greek for “light-bearing”. Keeping it secret, Brand would spend the rest of his life attempting to transmute this phosphorescent substance into gold, unsuccessfully.

This misguided search produced some of the first repeatable experiments that allowed us to evolve past the ideas upon which alchemy was based. The careers of most court alchemists ended abruptly when they could not produce the gold demanded by their employers and could not skip town fast enough. The shrewd alchemist would ask for a payment up front, then quite often melt a small amount of gold, and paint lead bricks; a technique that worked, until the “gold” bricks were cut open. As we failed to produce gold, however, we succeeded in creating for ourselves a real science. Across the Middle Ages, the population of elements roughly doubled: from seven to thirteen. Most of these discoveries involved the extraction of new metals from ores, and yielded zinc, arsenic, antimony, and bismuth. The end of the Middle Ages brought with it the recognition of the formation of gases, or “airs”, as they were known at the time.

By the beginning of the 1600s, we were unable to recognize if a gas was consumed by a chemical reaction. If a gas was produced, it was allowed to escape. This meant that we couldn’t keep accurate track of the comings and goings of a chemical system. It wasn’t until Joseph Priestley collected a gaseous product by displacing water from an inverted bottle. Reacting metal filings with an acid created a gas that would explode violently; heating a “calx” of mercury (mercury oxide) would evolve a gas that made a candle flame glow brightly. Priestley has begun to separate the “element” of air into multiple components. At nearly the same time, French tax collector Antoine LaVoisier performed a series of experiments proving that a reaction that produced gas could also be reversed, using the gas previously created, and maintaining a balance of mass.

“Nature,” LaVoisier might have said, “is a closed system. Passing steam through a gun barrel buried in hot coals causes the gun barrel to rust. A gas, given off, when collected, weighs less than the steam that entered the barrel. This gas must be different than the water vapor, as it is flammable. Carefully measuring the rusty gun barrel shows that the barrel weighs more than the clean iron barrel beforehand. The mass gained by the barrel, and the mass lost by the airs, is the same mass. What the air loses, the barrel gains.” It is this series of experiments that earned LaVoisier the title of “Father of Modern Chemistry”. It was his job as a tax collector that led to his premature beheading as a symbol of the French aristocracy during the Revolution in May of 1794.

Though experiments would continue to isolate new elements from natural sources, no whole-scale system of organization was yet in place. With only 4 Elements: Earth, Air, Fire, and Water; there was no need for a complicated organizational scheme. But as our knowledge of the “real” elements increased, we started noticing some pesky similarities.

Answer 3

Part III: Early "Laws"
By 1817, German chemist Johann Dobereiner pointed out that there were sets of elements whose masses exhibited a strange linkage: the elements lithium, sodium and potassium were chemically “related”. All three combined in a 2:1 ratio with oxygen; all three existed in 1:1 relationship with elements like chlorine; and then there was the issue of mass. Lithium has a relative mass of 7. Potassium has a relative mass of 39. Average these two together, and you get 23, the average mass of sodium. This trio of elements was not alone on the list of known elements: calcium, strontium, and barium; phosphorus, arsenic, and antimony; sulfur, selenium, and tellurium; chlorine, bromine, and iodine all showed this mysterious linkage of average masses. This was one of the first indications that mass would be important when arranging the atoms into a systematic table.

Dobereiner published his findings in 1817, calling it a “law of triads”. The problem was that, of the approximately 50 elements known at the time, this “law” only seemed to apply to 15, less than one third of the total. As a result, this “law” of triads was put aside, but the importance of mass on the periodic table was demonstrated.

Through the next 50 years, scientists would continue to discover new elements in some rather exotic places. Working with samples of cerium oxide and cerium sulfate, Swedish chemist Carl Mosander discovered impurities within the ore samples whose properties did not match any known metal. By 1839, he suggested the name “lanthanum”, from the Greek meaning “to lie hidden”, as cerium had been discovered and confirmed in 1803. The previously ignored “impurities” in the cerium had been lanthanum oxides hiding the whole time. Mosander would go on to isolate two more elements, erbium and terbium, from samples of the mineral yttria.

The Swedish town of Ytterby has a unique honor among the elements of the periodic table: no fewer than four elements take their names from the tiny village, and three more have roots that trace back to minerals first mined in the quarry nearby (Kean). Read Sam Kean’s The Disappearing Spoon for a much more thorough treatment of the discovery of these elements.

By 1864, German chemist Julius Meyer published a table listing the known 44 elements by valency, noting that elements with similar valency (charge) also shared similar chemical and physical properties. An English chemist, William Odling, made a similar table hinting at the idea of a periodic “law”, but did not, apparently, pursue it. In 1865, English chemist John Newlands organized the 56 known elements into 11 groups, increasing by mass, and based on similar properties. They resembled Dobereiner’s triads, but Newlands’ organization hinted that, by some happenstance, every 8 elements were related. Newlands’ “law of octaves” was ridiculed by his contemporaries.

By 1869, the population of elements had ballooned to over 60, and no two chemists had the same system to catalog them. Geologists kept digging up new minerals and there had to be some place to put them all. Chemists were at a loss as to where to put these new discoveries. In March of that year, Russian chemist Dmitri Mendeleev presented to the Russian Chemical Society The Dependence between the Properties of the Atomic Weights of the Elements. In this, he outlined a table that made some bold predictions.

Most prior tables had made the assumption that additional elements would be added on at the end of the table. Mendeleev organized his table by both atomic mass and the similarities in properties. He left GAPS where no known element fit the existing pattern. Arsenic directly followed zinc in most element tables of the time, but the properties of arsenic placed it firmly in the nitrogen family. Mendeleev’s great breakthrough arose (solely my opinion) out of his humility. He recognized that, perhaps, we didn’t have all the information yet, and any attempt to make nature fit a desired pattern was a bad fit. This swapping occurred with copper and nickel, as well.

In 1871, Mendeleev argued that an as-yet undiscovered element, which he named eka-aluminum (eka- being the Sanskrit prefix for “one”) would fill in the gap below aluminum and to the right of zinc in his table of elements. Using the patterns of his early table, Mendeleev predicted, with fair certainty, the density, melting point, oxidation state, formulas of both oxides and chlorides, and solubilities in acids and alkalis of what would be discovered in 1875 by deBoisbaudran and named gallium.

A chemist that makes one successful prediction of this nature could very well be considered a success. Mendeleev made four: scandium (Z=21), gallium (Z=31), germanium (Z=32) , and technetium (Z=43). Mendeleev’s predictions threw the figurative gauntlet at the geologists of the time. Previously, geologists were the people challenging chemists to find a proper place for their newly-discovered elements. Mendeleev’s predictions reversed that challenge. Mendeleev effectively says to the geologists “there’s an element out there with these properties; YOU GO FIND IT!”. It is no surprise that in1963 the International Union of Pure and Applied Chemistry (IUPAC) approved the name of element 101 as “Mendelevium”

The success of Mendeleev’s table was hindered by an incomplete understanding of why the elements went in the orders they did. Prior to 1913, the “ranking number” of the atoms was determined by preference: hydrogen has the lightest mass, so it gets number 1, and so on. By bombarding metal samples with high energy X-rays, Moseley discovered a relationship between the wavelengths of the X-rays and a small whole number that varied from element to element. That small whole number would be called the atomic number , and equal to the number of protons inside the nucleus of each atom. This discovery verified the “swapping” of elements as Mendeleev had done in order to preserve the periodicity of the elements’ properties (say THAT five times). Tragically, Moseley volunteered as a communications officer in the British Army, was sent to the ill-fated region of Gallipoli, Turkey, and was shot and killed on August 10, 1915. He was 27 years old.

Answer 4

Moseley’s discovery verified the “swapping” of elements as Mendeleev had done in order to preserve the periodicity of the elements’ properties (say THAT five times). A close look at a modern periodic table shows that mass typically increases as the atomic number increases: hydrogen has an average atomic mass of 1.007amu; helium 4.003; lithium 6.941; and so on. This pattern continues almost uneventfully, but for a few exceptions. Cobalt (Z=27) is slightly heavier than nickel (Z=28). Argon (Z=38) is heavier than potassium (Z=39), and tellurium (Z=52) is heavier than iodine (Z=53). Mendeleev placed iodine preferentially based on its similarities to the other halogens, even though tellurium outweighs iodine by 0.7amu, on average. What Mendeleev could not do was explain why. Moseley’s arrangement validated Mendeleev’s “hunch”.

By 1913, new experiments with electricity were trying to answer yet more questions about the structure of atoms themselves. Emission spectra for the elements were observed, and their origins hypothesized by the greatest minds of the early 20th Century. Planck, Bohr, deBroglie, Schrodinger, Heisenberg, Pauli, Einstein, and more worked to advanced our picture of the atom, and gave rise to the field of Quantum Mechanics. Probability distributions, wave functions, and orbitals offered a more accurate, if more head-scratching, explanation of the properties of elements. As Mendeleev may have predicted, new elements need a place on the Periodic Table, which was becoming larger and more complicated.

Mendeleev’s Periodic Table still maintained eight columns, a trait that persisted until the early 1920’s, when H.G. Deming proposed an 18-column table, moving the transition metals to their own home between the Main Group elements. The last “family” of elements to move were the Rare Earth Elements, so named not because of their scarcity, but because of the difficulties in chemically separating mixtures of their ores (Greenberg). According to USGS data, the two rarest rare earth metals, thulium and lutetium, are nearly 200 times more common than gold in the earth’s crust. The early half of the 20th Century was a wild time for the Rare Earths, and teams from countries all over Europe vied for the rights to place their nationalistic pride on the ever-expanding Periodic Table.

But the same problem that plagued earlier tables continued. The properties of these newly discovered elements did not match the properties of their “older” siblings; the elements in higher periods of the same group. The properties of Cerium (Z = 58) did not align with the properties of the transition metals titanium (Z = 22) and zirconium (Z = 40). The melting points of these new “rare” earths were significantly lower than their places on the table would seem to indicate. Titanium melts at 1943K, Zirconium at 2125K, but Cerium a paltry 1071K by comparison. Vanadium (Z = 23) melts at 2163K, Niobium (Z = 41) at 2740, but Praseodymium (Z = 59) melts at just 1204K. Elements 89 and larger continued this mysterious trend.

In 1945, rogue American physicist Glenn Seaborg made a radical suggestion. Having already been the principal or co-discoverer of ten different trans-uranium elements, Seaborg suggested that the placement of the actinide series was incorrect. He proposed that the actinides were filling another series of f-orbitals, like the lanthanides above them. As such, they should be moved from beneath the transition metals to a more fitting location underneath the lanthanides. When his colleagues suggested he not publish such a radical departure from periodic canon, Seaborg was said to reply “since I don’t have much of a reputation to lose, I might as well try”.

Since then, the periodic table has gone through probably 700 alternative variations. Probably the most popular of which is Theodore Benfey’s spiral table of 1960 (Emsley). Three-dimensional tables that take on spiral or helical patterns are also popular, showing the cyclical nature of the properties of the elements. For more information, see the texts below.

Aldersley-Williams, H.; Periodic Tales: a Cultural History of the Elements, from Arsenic to Zinc

Chang, R.; Chemistry: 9th Ed.

Coelo, P.; The Alchemist

Emsley, J; Sharp, R.; The Periodic Table: Top of the Charts; The Independent; November 2014

Gonick, L.; Criddle, C.; Cartoon Guide to Chemistry

Greenberg, A.; From Alchemy to Chemistry in Picture and Story

James, M.; The Life and Work of Charles James; UNH Magazine; Fall 2010

Kean, S.; The Disappearing Spoon