OAT Practice Test 4

An Anomaly in the Third Decimal Place

¹In the early 1890s, the English physicist Lord Rayleigh set himself a task that seemed more tedious than adventurous: measuring the densities of common gases with greater accuracy than anyone had achieved before. His motive was a long-standing question in chemistry. Early in the nineteenth century, the physician William Prout had proposed that the atomic weights of all elements were whole-number multiples of the atomic weight of hydrogen, suggesting that hydrogen might be the building block of all matter. Many measured atomic weights came tantalizingly close to whole numbers, but others did not, and precise gas densities offered a way to test whether the discrepancies were real or merely the product of imperfect measurement.

²Oxygen and hydrogen behaved as expected. Nitrogen did not. Rayleigh prepared nitrogen in two ways. In one, he took ordinary air and removed its oxygen, water vapor, and carbon dioxide, leaving what was then believed to be pure nitrogen. In the other, he obtained nitrogen chemically, by decomposing nitrogen-containing compounds such as ammonia. The two samples should have been identical. Yet a flask filled with nitrogen from the air was consistently heavier than the same flask filled with nitrogen from chemical sources, by roughly half of one percent. The difference appeared only in the third decimal place of his measurements, but it was far larger than the variation among repeated trials of either method.

³A less careful investigator might have averaged the two figures, or blamed an unidentified flaw in the apparatus, and moved on. Rayleigh instead treated the discrepancy as a fact demanding explanation. In 1892 he published a short letter in a scientific journal describing the puzzle and inviting chemists to suggest a cause. Several possibilities presented themselves. The chemical nitrogen might be contaminated with a lighter gas such as hydrogen. Some of its molecules might have split into individual atoms, which would lower its density. Alternatively, the atmospheric nitrogen might contain a heavier form of nitrogen, perhaps a three-atom molecule analogous to ozone, the three-atom form of oxygen. A fourth possibility, that the air contained an unknown gas, seemed to many the least likely of all, since the composition of air had been studied for more than a century.

⁴Rayleigh tested these explanations in turn and found none of the first three adequate. Nitrogen prepared from several different compounds gave the same low density, which would have been an unlikely coincidence if each sample carried its own impurity, and his samples showed no sign of the gradual change in density that unstable split or heavy molecules would be expected to produce. By 1894 he was joined by the chemist William Ramsay, who approached the problem from the opposite direction. If air contained something besides nitrogen, then removing all of the nitrogen should leave that something behind.

⁵Ramsay passed atmospheric nitrogen back and forth over heated magnesium, a metal that combines with nitrogen to form a solid nitride. With each pass the volume of gas shrank and, tellingly, the density of what remained rose. When no further shrinkage occurred, a small residue was left that refused to react with magnesium or with anything else the investigators tried. Rayleigh, working separately, reached a similar residue by a different method, repeatedly passing electric sparks through air mixed with extra oxygen so that the nitrogen combined into oxides that could be absorbed. His method closely followed one used in 1785 by an experimenter who had noted a stubborn bubble of unreacted gas, no more than about one part in 120 of the original volume, and had left it unexplained. The new residue amounted to roughly one percent of the volume of air. Its spectrum, viewed through a spectroscope, showed bright lines that matched those of no known element.

⁶The discovery was announced in 1894, and the new gas was named argon, from a Greek word meaning "lazy" or "idle," in recognition of its refusal to combine with other substances. Its inertness, however, created a problem of its own. For most new elements, chemists determined the atomic weight by analyzing compounds, but argon formed none. Rayleigh and Ramsay turned instead to physics. By measuring the speed of sound in the gas, they could determine how its molecules stored energy, and the result indicated that argon, unlike nitrogen or oxygen, consisted of single atoms rather than pairs. Its atomic weight was therefore about 40.

⁷That figure placed argon awkwardly in Dmitri Mendeleev's periodic table, which arranged the elements in order of atomic weight. An element weighing about 40 should have fallen beside potassium and calcium, but those positions were already filled by elements whose chemical behavior suited them perfectly, and argon's properties resembled neither. Some chemists, Mendeleev among them, initially suggested that argon might not be an element at all but the long-sought three-atom form of nitrogen, which would have a molecular weight of about 42. Others suspected that the periodic system itself was flawed.

⁸The way forward came from an unexpected direction. In 1895 Ramsay, searching for natural sources of argon, treated a uranium-bearing mineral with acid and collected the gas it released. Its spectrum contained a brilliant yellow line that did not belong to argon. It matched a line that astronomers had observed in the light of the Sun during a solar eclipse in 1868 and had attributed to a hypothetical element, called helium after the Greek word for the Sun, that was thought to exist only there. Helium, like argon, was a monatomic and unreactive gas, but it was very light, with an atomic weight near 4. Two such elements suggested something more than an isolated curiosity: perhaps there was an entire family of inert gases, forming a new column of the periodic table, with gaps waiting to be filled.

⁹Ramsay and his assistant Morris Travers set out to fill them. By then it had become practical to liquefy air in quantity. Allowing liquid air to boil away slowly and collecting the fractions that evaporated at different temperatures, they isolated three more new gases in 1898: krypton ("hidden"), neon ("new"), and xenon ("stranger"). Each showed its own distinctive spectrum, and their atomic weights fitted a regular sequence. The new column sat neatly between the halogens, such as chlorine, and the alkali metals, such as potassium. Far from overturning the periodic system, the inert gases became one of its most striking confirmations, since an entire group had been anticipated and found within a few years.

¹⁰One difficulty remained. Argon, with an atomic weight near 40, is heavier than potassium, whose atomic weight is about 39, yet argon's properties required it to come first. The inconsistency was resolved only in 1913, when the physicist Henry Moseley showed from X-ray spectra that each element has a characteristic atomic number, now understood as the number of protons in its nucleus. Ordered by atomic number rather than by weight, argon falls naturally before potassium. The reversal that had troubled chemists turned out to reflect the different numbers of neutrons carried by the most common forms of the two elements, a particle that would not be discovered for nearly another two decades.

¹¹In 1904 Rayleigh received the Nobel Prize in Physics for his investigations of gas densities and his discovery of argon, and Ramsay received the Nobel Prize in Chemistry for his discovery of the inert gases and his determination of their place in the periodic system. The name "inert" did not last. In 1962 a chemist succeeded in preparing a compound of xenon, compounds of krypton followed, and the group is now usually called the noble gases. The episode is often retold as a lesson in scientific method. A discrepancy of half a percent, which could easily have been dismissed as error, led to the discovery of an entire family of elements, because an investigator trusted his measurements enough to take a small anomaly seriously.