INTERCHAPTER K The Noble Gases When an electric discharge is passed through a noble gas, light is emitted as electronically excited noble-gas atoms decay to lower energy levels. The tubes contain helium, neon, argon, krypton, and xenon. University Science Books, ©2011. All rights reserved. www.uscibooks.com Title General Chemistry - 4th ed Author McQuarrie/Gallogy Artist George Kelvin Figure # fig. K2 (965) Date 09/02/09 Check if revision Approved K. THE NOBLE GASES K1 2 0 Nitrogen and He Air P Mg(ClO ) NaOH 4 4 2 noble gases 4.002602 1s2 O removal H O removal CO removal 10 0 2 2 2 Ne Figure K.1 A schematic illustration of the removal of O2(g), H2O(g), and CO2(g) from air. First the oxygen is removed by allowing the air to pass over phosphorus, P (s) + 5 O (g) → P O (s). 20.1797 4 2 4 10 2s22p6 The residual air is passed through anhydrous magnesium perchlorate to remove the water vapor, Mg(ClO ) (s) + 6 H O(g) → Mg(ClO ) ∙6 H O(s), and then through sodium hydroxide to remove 18 0 4 2 2 4 2 2 the carbon dioxide, NaOH(s) + CO2(g) → NaHCO3(s). The gas that remains is primarily nitrogen Ar with about 1% noble gases. 39.948 3s23p6 36 0 The Group 18 elements—helium, K-1. The Noble Gases Were Kr neon, argon, krypton, xenon, and Not Discovered until 1893 83.798 radon—are called the noble gases 2 6 4s 4p and are noteworthy for their rela- In 1893, the English physicist Lord Rayleigh noticed 54 0 tive lack of chemical reactivity. Only a small discrepancy between the density of nitrogen Xe xenon and krypton are known to obtained by the removal of oxygen, water vapor, and carbon dioxide from air and the density of nitrogen 131.293 enter into chemical compounds, 5s25p6 and even then only with the two prepared by chemical reaction, such as the thermal 86 0 most electronegative compounds, decomposition of ammonium nitrite: Rn fluorine and oxygen. The principal (222) source of the noble gases, except for NH4NO2(s) → N2(g) + 2 H2O(l ) 6s26p6 helium, is the atmosphere, where they are thought to have arisen as One liter of nitrogen at 0°C and 1 atm obtained by by-products of the decay of radioac- the removal of all the other known gases from air has tive elements in the earth’s crust. Because of its low a mass of 1.2572 grams (Figure K.1), whereas one liter mass, however, helium escapes from the earth’s atmo- of dry nitrogen obtained from ammonium nitrite has sphere into outer space. Properties of the noble gases a mass of 1.2505 grams under the same conditions. are given in Table K.1. The data in Table K.1 nicely This slight difference led Lord Rayleigh to suspect illustrate trends in physical properties with increas- that some other gas was present in the sample of ing atomic size. nitrogen from air. TABLE K.1 Properties of the noble gases Atomic Atomic Ionization Concentration Melting Boiling Element Symbol mass radius/pm energy/MJ·mol−1 in air/ppm point/°C point/°C helium He 4.002602 32 2.3723 5.2 — −268.93 neon Ne 20.1797 69 2.0806 18.2 −248.609 −246.053 argon Ar 39.948 97 1.5205 9340 −189.36 −185.847 krypton Kr 83.798 110 1.3507 1.1 −157.38 −153.34 xenon Xe 131.293 130 1.1704 0.08 −111.745 −108.09 radon Rn (222) 145 1.0370 ~ 1 × 10−15 –71 –61.7 K2 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly The English chemist William Ramsay found that if hot calcium metal is placed in a sample of nitrogen obtained from air, about 1% of the gas fails to react. Pure nitrogen should react completely. Because of the inertness of the residual gas, Ramsay gave it the name argon (Greek, idle). Ramsay then liquefied the residual gas and, upon measuring its boiling point, discovered that it consisted of five compo- nents, each with its own characteristic boiling point (Table K.1). The component present in the greatest amount retained the name argon. The others were named helium (sun), neon (new), krypton (hidden), and xenon (stranger). Helium was named after the Greek word for sun (helios) because its presence in Figure K.2 The two discoverers of the noble gases. the sun had been determined earlier by spectro- (left) Sir William Ramsay was a professor of chemistry at the scopic methods. All the noble gases are colorless, University College of London and (right) Lord Rayleigh, born John William Strutt, was Cavendish Professor of odorless, and relatively inert. For their work in discov- Physics at Cambridge University. ering and characterizing an entire new family of ele- ments, Rayleigh received the 1904 Nobel Prize in Physics and Ramsay received the 1904 Nobel Prize Chapter 2 that α-particles are energetic helium nuclei. in Chemistry (Figure K.2). An estimated 3000 metric tons of helium are formed each year in this way. Naturally occurring helium is K-2. Helium Is the Second Most Abundant trapped in nonpermeable rock layers beneath the Element in the Universe earth. Helium is obtained by separation from natural gas (which is also trapped in such layers) using frac- Helium is the second lightest element and the second tional distillation. The largest producer of helium is most abundant element in the universe, after hydro- the United States, which has extensive quantities of gen. Despite the fact that helium is denser and hence helium in its natural gas deposits. Starting in 1925, has less lifting power than hydrogen, it is used in when the use of lighter-than-air aircraft yielded a mil- lighter-than-air aircraft because it is nonflammable. itary advantage, and continuing through the space Helium is used in welding to provide an inert atmo- race and the cold war, when helium was needed as sphere around the welding flame and thus reduce a coolant for military and space applications, the corrosion of the heated metal. An inert helium envi- United States government maintained a strategic ronment is used for growing silicon and germanium helium reserve. In 1996 the U.S. government began crystals in the semiconductor industry. Helium is phasing out this reserve. used as an inert carrier gas in gas chromatography The speed of sound in helium is nearly three (an analytical method used to separate mixtures). times that in air because of its low atomic mass (see It is also used to replace nitrogen in deep-sea-diving Chapter 13). For this reason, if helium is inhaled, say, breathing mixtures to help reduce the formation of from a small balloon, it can cause a temporary change nitrogen bubbles in the blood, a condition known as in the fundamental frequency of the vocal cavity, the bends (see Section 16-7). making the voice sound high-pitched. However, this Because helium boils at 4.22 K, liquid helium is must be done cautiously as helium is an asphyxiant often used as a coolant for superconducting magnets and inhaling too much helium and no oxygen can such as those used in MRI (magnetic resonance imag- cause respiratory distress or even death. ing) and in other cryogenic applications. For exam- ple, the recently constructed Large Hadron Collider uses 96 metric tons of liquid helium for cooling. On earth helium is produced from the radioac- Never inhale a gas directly from a high-pressure tive decay of α-emitters (alpha-emitters) such as nat- tank, as lung damage can result. urally occurring uranium and thorium. Recall from University Science Books, ©2011. All rights reserved. www.uscibooks.com K. THE NOBLE GASES K3 K-3. Excited Neon Gas Emits an TABLE K.2 Composition of the earth’s atmosphere Orange-Yellow Glow below 100 km Neon is the fifth most abundant element in the uni- Major Content in fraction of total verse, but is relatively rare on earth due to its low constituents molecules (and percent by mass) mass, which means that, like helium, it also escapes nitrogen (N ) 0.7808 (75.51%) from the upper atmosphere into space (although at 2 oxygen (O ) 0.2095 (23.14%) a much slower rate than helium). When placed in 2 a discharge tube, neon emits an orange-yellow glow argon 0.0093 (1.28%) that penetrates fog very well (Frontispiece). Neon is water vapor 0–0.04 used in neon signs, which are essentially discharge Minor Content in parts tubes filled with neon or a gas mixture containing constituents per million (ppm)* neon. In fact, because of its high cost, very few “neon” signs actually contain pure neon and some contain carbon dioxide 385 ppm (2009 data) no neon at all. Neon has traditionally been used in a neon 18 ppm variety of electronics such as vacuum tubes and televi- helium 5 ppm sion tubes and in some cryogenic applications. Neon methane 2 ppm is also used in helium-neon lasers, which operated as bar-code scanners and optical disk readers before the krypton 1 ppm development of less costly diode lasers. hydrogen (H2) 0.5 ppm dinitrogen oxide 0.5 ppm K-4. Argon Is the Third Most Abundant Gas xenon 0.1 ppm in Our Atmosphere and Has a Wide Range *The unit ppm denotes parts per million parts, for example, of Industrial Uses 385 ppm of carbon dioxide means that 385 of each 1 million molecules are CO . Argon is the most plentiful and least expensive 2 noble gas.