Finding Order Among the Elements

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Finding Order Among the Elements

Overview

The periodic table is one of the most fundamental tools of chemistry. It summarizes information about each element and reveals how the elements are chemically related to one another. The first widely accepted periodic table was published in 1869 by a chemistry professor named Dmitri Mendeleyev. He began work on his table hoping to help his students to learn about the elements. He ended up by creating a classification system that helped chemists predict new elements and that led to the discovery of the particles that make up atoms.

Background

One of the key developments that led to the periodic table was the determination of the atomic weights of the elements. In 1805 English chemist John Dalton (1766-1844) stated that every atom of an element has the same weight. This idea implied that it would be possible to measure the atomic weights of the elements. (It is now known that Dalton's hypothesis is incorrect; not all atoms of an element have the same weight. Atomic weights can be measured, but they usually represent an average.)

In 1809 Joseph Louis Gay-Lussac (1778- 1850) proposed that when gases undergo a chemical reaction, they do so in simple whole number ratios of their volumes. For example, he showed that dinitrogen oxide (N₂O) was formed from two volumes of nitrogen to one volume of oxygen. Jöns Jacob Berzelius (1779-1848), a Swedish scientist, used the ideas of Dalton and Gay-Lussac to determine the atomic weights of the 69 elements known at that time. He did so by measuring the relative volume of oxygen with which various elements could combine. He could then infer the atomic weight of the elements from these measurements of volume. He published quite accurate tables of atomic weights in 1818 and 1826.

Berzelius was also largely responsible for the fact that the ideas of Amedeo Avogadro (1776-1856) went unnoticed for almost half a century. In 1811 Avogadro had hypothesized that equal volumes of gases contain equal numbers of molecules. If this were the case, the relative atomic weights of gases would be fairly simple to determine. For instance, if one liter of oxygen weighed approximately 16 times the weight of one liter of hydrogen, then it could be concluded that one atom of oxygen weighs about 16 times as much as one atom of hydrogen. Avogadro also proposed that two atoms of an element may combine to form one molecule of gas, as in oxygen (O₂) and nitrogen (N₂). It was on this point that Berzelius strongly disagreed with Avogadro. As a result Berzelius and many other scientists of the time wrote incorrect formulas for many compounds. For instance, Berzelius wrote the formula of water as HO rather than H₂O. These incorrect formulas sometimes resulted in incorrect measurements of atomic weight.

In 1858 Italian chemist Stanislao Cannizzaro (1826-1910) showed that the atomic weights of the elements in a molecule could indeed be calculated by applying Avogadro's hypothesis. In addition, he helped to define the difference between atomic weight (that of an atom, such as H) and molecular weight (that of a molecule, such as H₂). Cannizzaro brought forward Avogadro's hypothesis at the first international meeting of chemists. This meeting, called the Karlsruhe Conference, was held in Heidelburg, Germany, in 1860. After Cannizzaro's presentation, Avogadro's hypotheses became widely accepted within a few years.

One of the scientists in attendance at the Karlsruhe Conference was Dmitri Mendeleyev (1834-1907). Mendeleyev was a Russian chemist who happened to be studying in Europe at the time. Mendeleyev made the acquaintance of Cannizzaro, from whom he obtained measurements of atomic weights and a familiarity with the ideas of Avogadro. After returning to Russia Mendeleyev began searching for a logical way to organize the elements. Eventually, he noticed that when he arranged the elements in order of increasing atomic weight, similar elements appeared at regular, or periodic, intervals. Mendeleyev used his observations to make a table that reflected this pattern.

In March 1869 Mendeleyev presented his table to the Russian Chemical Society. A revised version followed two years later. The atomic weight of the elements in his table increased from left to right across each row, or period. Each column, or group, of the table contained elements with similar properties. The heading of each column indicated the valency of the elements in that group. An element's valency is responsible for the way in which it combines with other atoms. Thus, Mendeleyev's table showed that an element's chemical properties, as seen through its valency, were a function of its atomic weight. Mendeleyev called this relationship the periodic law, and his table became known as the periodic table.

Impact

As he worked on his table, Mendeleyev began to suspect that there were elements that had yet to be discovered. (In fact, at the time his original table was published, scientists had described only 69 of the 112 chemical elements known today.) He left blanks in his table to accommodate these undiscovered elements and even made predictions of their properties based on those of neighboring elements. In all, Mendeleyev made predictions for ten new elements, seven of which eventually proved to be correct.

Three of these elements, which Mendeleyev called eka-boron, eka-aluminum, and eka-silicon, were discovered within 20 years of the publication of his first table. In 1875 Paul Émile Lecoq de Boisbaudran (1838-1912), a French chemist, discovered the element gallium, which turned out to be Mendeleyev's eka-aluminum. Mendeleyev had predicted an atomic weight of 68 and a specific gravity of 5.9 for this element. (The specific gravity of an element is its density divided by the density of water.) When measured experimentally, gallium's atomic weight is 69.72 and its specific gravity is 5.94—a close match to Mendeleyev's predictions. Lars Fredrick Nilson discovered scandium in 1879, which Per Teodor Cleve (1840-1905) identified as Mendeleyev's eka-boron, and Clemens Winkler (1838-1904) discovered germanium, Mendeleyev's eka-silicon, in 1886. Germanium's specific gravity was measured at 5.469, which agreed nicely with Mendeleyev's prediction of 5.5. However, some of the chemical reactions Mendeleyev predicted for these three elements turned out not to be as accurate. So although these discoveries confirmed Mendeleyev's system and showed that it could be used as a practical research tool, they also demonstrated the need for obtaining experimental data.

Despite his successful predictions, Mendeleyev's periodic law did not always hold true. For instance, according to his law it would be impossible for two elements to have the same atomic weight. However, the elements nickel and cobalt as well as the elements ruthenium and rhodium were assigned identical atomic weights in his table. In addition, the difference in atomic weight between consecutive elements in the table was inconsistent. The smallest difference appeared to be about 0.25 unit and the greatest seemed to be about 4 to 5 units. If an element's properties were truly dependent on its atomic weight, it seemed as if atomic weight should change by the same amount between consecutive elements. Another problem was that Mendeleyev had to switch some elements in order to make them fit his pattern. For example, he positioned tellurium to the left of iodine even though iodine was assigned an atomic weight of 127 and tellurium was assigned an atomic weight of 128.

Mendeleyev's table also raised the question of what is added to an element when its atomic weight increases. The pattern Mendeleyev had discovered in the atomic weights of the elements gave support to the idea that the elements had the same origin. According to this idea, each of the elements is made from the same basic particles—the same "stuff"—but in a unique arrangement or quantity. For instance, atoms of gold and silver would be made of the same types of particles, but gold atoms would have a different arrangement or a different number of particles than those of silver.

In the late 1800s several scientists began to speculate about the types of subatomic particles that might exist. However, many other scientists had problems with the idea of so-called "ultimate particles." They argued that atoms of elements with a high atomic weight would have to consist of hundreds of similar particles, which was thought to be a highly unstable situation. In addition, it was argued that if there were particles smaller than atoms, one element might be able to change into another, which was generally believed to be impossible. This debate was not settled until the twentieth century, when the existence of subatomic particles (protons, neutrons, and electrons) was finally confirmed. (Scientists now know that strong forces within an atom's nucleus allow similarly charged protons to be packed closely together, and in some cases, radioactive decay does result in one element changing into another.)

Once subatomic particles were discovered, it was realized that the periodic law holds true for atomic number rather than atomic weight. The atomic number of an element is equal to the number of protons or the number of electrons in a neutral atom. It is the number of electrons that governs an atom's chemical and physical properties, and the modern periodic table is based on increasing atomic number rather than increasing atomic weight.

When the elements are arranged by atomic number, consecutive elements differ by one proton and one electron. The difference in neutrons between consecutive elements, however, does not follow a set pattern. It is for this reason that Mendeleyev's periodic law does not always hold true. Atomic weight is based in part on the number of neutrons in an atom, so a law based on atomic weight will sometimes be inconsistent.

The discovery of the noble gases also posed a temporary threat to the periodic table. William Ramsay (1852-1916) and Lord Rayleigh (1842-1919) discovered the noble gas argon in 1894. Mendeleyev doubted that the men had actually discovered a new element, mainly because there was no space for it in his periodic table. Ramsay suggested that argon should be placed in its own group after chlorine and before potassium. However, argon's atomic weight was less than that of potassium, so this idea was not seen as acceptable at the time. Within four years, however, Ramsay had isolated the noble gases krypton, neon, and xenon. (Helium had been discovered earlier by examining the light of stars.) These elements helped to fill up the rightmost column of the table, showing that Ramsay's original suggestion was correct and that the essence of the periodic law was not violated. Today, the periodic table contains spaces for 118 elements, and it is used in classrooms and laboratories throughout the world.

STACEY R. MURRAY

AN UNFORTUNATE METAPHOR>

In 1864, five years prior to the publication of Mendeleyev's table, an English chemist named John Newlands suggested that when the elements were arranged according to atomic weight, similar chemical properties would be found with every eighth element. He named his hypothesis the "law of octaves," comparing the pattern of eight elements to that of the eight notes of the musical scale. The basic idea behind his hypothesis—a connection between atomic weight and chemical properties—was valid, as Mendeleyev was soon to show. However, he lacked sufficient evidence to support his claim, and his comparison of elements to musical notes was ridiculed as being "mystical" and unscientific. One chemist even asked him sarcastically if he found similar patterns by arranging the elements alphabetically. However, Newlands' idea was not nearly as farfetched as that of another chemist, who claimed to have found a connection between atomic weight and the distances of the planets from the sun.

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