The periodic table is the chemist’s playground and an icon of science that’s found on science classroom walls by the millions all over the country, all around the world. It’s one of the most powerful conceptual tools in all of science, yet it didn’t spring forth all at once.
Long before the existence of atoms had been proven—that wasn’t until the 20th century, after all—there still were chemical researchers all over the world who were looking to find useful compounds. And some of these substances that they found seemed to be more fundamental than others. These “elements” could not be broken down into other substances by any normal physical means. They seemed to be more fundamental.
Ancient cultures knew of at least 10 different chemical elements. Most of these were found in their native state as metals, including copper, silver, gold, platinum, lead, mercury, and tin.
Ancient cultures knew of at least 10 different chemical elements. Most of these were found in their native state as metals, including copper, silver, gold, platinum, lead, mercury, and tin. You also have nonmetallic elements that occur in nature; carbon comes from charcoal and sulfur from volcanoes.
Then we come to the 12th through the 16th centuries—the period of alchemy—where various scholars learned to separate a few other elements by using intense fires. The new elements discovered during this period include arsenic, zinc, bismuth, and phosphorous.
Between 1735 and 1805, chemists devised new techniques to purify and characterize more than two dozen new elements. Many of the new elements were discovered by studying minerals through wet chemical analysis, where you dissolved and broke apart the minerals; or by blowpipe analysis, where you used an intense hot flame to melt and fuse various parts of the minerals. The important elements nickel, cobalt, magnesium, chromium, molybdenum, tungsten, uranium—they were all discovered during this period. Also during this period was the first isolation of gaseous elements: hydrogen, nitrogen, and oxygen.
Mendeleev Brings Order to the Elements
Order was given to the wide and growing range of chemical elements by the Russian chemist Dmitri Mendeleev. Many of his earliest publications relate to the chemical analysis of minerals. It was this work, along with his interest in teaching chemistry, that led him to search for systematics in the patterns and properties of the different chemical elements. Mendeleev knew of 63 different chemical elements, and that’s when he began working on his famous table in the 1860s.
It turns out that chemical elements have a whole number of measurable properties, and this provides the basis for grouping the elements in various rows and columns. You have relative weights; you can measure the relative weights by observing ratios of the weights when compounds are decomposed. When water is decomposed by electricity, for example, it produces two parts hydrogen to one part oxygen gas by volume, 1:8 by weight.
By measuring the decomposition of many different compounds, the relative weights of many different elements can be determined. If hydrogen has weight 1, for example, Mendeleev found that sodium has weight 23, calcium has weight 40, and barium has weight 137, for example. All the elements then can be arranged in sequence according to increasing weight.
Elements also displayed other systematic properties. During electrolysis, for example, a mineral or other compound is dissolved in acid. The two electrodes separate out these elements. What was found was that a few elements—including oxygen and chlorine and bromine—always went to the positive electrode. Metals were always deposited at that negative electrode. And so you can collect the metals and determine their properties.
So, for example, if you take silver chloride and you dissolve it, the chlorine gas comes bubbling up off the positive side. Then the silver metal is plated onto the negative anode. In fact, this is the principle for silver plating and other kinds of metal plating by electrical methods, which are called electroplating, in a general sense. You do it just using a battery and dissolved metals in the solution.
It turns out that the amount of voltage required to initiate this process is also one of the periodic properties. As you go across certain sets of elements, you see there’s a systematic pattern. You need more voltage for some elements than others.
Long before Mendeleev’s work, chemists had recognized that some groups of elements show striking similarities, striking patterns.
Long before Mendeleev’s work, chemists had recognized that some groups of elements show striking similarities, striking patterns. This is true for their physical appearance, for their chemical properties, and for other sorts of behavior. Furthermore, for each of these groups of related elements, the physical and chemical properties seemed to somehow change systematically with their weight.
Let me give you a few examples of these periodic properties. The elements lithium, sodium, potassium, and rubidium, for example, are all soft, silvery metals. They’re called the alkaline metals and all react violently with halogens like chlorine.
Another group of elements is called the alkaline earth metals. These include beryllium, magnesium, calcium, and barium. The relative weights of these four silvery metals are nine 9 to 24 to 40 to 88. That’s an odd sequence of numbers, but there seems to be some sort of similarity. It’s not an identical pattern, but there’s certainly something very close there. Mendeleev noticed that sort of thing as well.
Still other elements, such as hydrogen and carbon and sulfur, were more difficult to group in any systematic way. There didn’t seem to be any other elements that were exactly like those, so Mendeleev had to be very puzzled when he filled this complete array of elements and tried to systematize them. Part of his success was his ability to focus on the most obvious patterns and then not worry too much about anomalies that seemed to creep in. This is an intuitive step in science—you have to know what details to ignore when you make your synthesis.
Leaving Room for the Unknown
Mendeleev forced the most unambiguous groups of similar elements into vertical columns. He also arranged his elements left-to-right and top-to-bottom by increasing weights. Thus, sodium was next to magnesium, potassium was next to calcium, and so forth.
He also left spaces for elements that appeared to be missing. For example, his table left room for unknown elements in positions 2, 10, and 18. There were also obvious gaps at element 21 and element 32, elements that had not been described and yet would appear to be present if the table were complete.
Scientists readily accepted the concept of the periodic table, not only because it systematized so much of what was known, but because it made very specific testable predictions about what was not known.