IT'S ELEMENTAL! fluoride—probably RnF2—but he and others were unable to confirm the existence of oxides or relatives of the perxenates. Thefirstionization potentials of the noble gases provide a measure of how firmly NEIL BARTLETT, UNIVERSITY OF CALIFORNIA, BERKELEY the outer electrons are held by the effecHE PERIODIC TABLE OF THE Kaye) to attempt (in 1933) a xenon fluo- tive nuclear charge. This hardness or softElements, as set out by Dmitry ride synthesis. That attempt failed. So mat- ness of the valence electron set correlates well with the physical properties ofthe gasMendeleyev or Lother Meyer, ters rested until 1962. had not allowed for the discovIt was the discovery of the remarkable es. Thefirstionization potentials are as folery of a group of eleoxidizing properties ofplatinum lows: He, 24.6; Ne, 21.6; Ar, 15.8; Kr, 14.0; ments between the highly elechexafluoride in making the salt Xe, 12.1; and Rn, 10.7 eV tronegative halogens and the Clearly, helium has the least polarizable 0 2 + PtF 6 _ that led (via the recogelectropositive alkalis. So the disnition that 0 2 and Xe have electron cloud. This accounts for its low covery of the unreactive monanearly the same first ionization melting and boiling points and its low soltomic gas argon by Lord Rayleigh potentials) to the oxidation of ubility in aqueous media, and hence its apand William Ramsay in 1895 xenon by PtF 6 . Later in 1962, plication in mitigating the bends as a dilucame as a total surprise. James Howard H. Claasen, Henry ent for oxygen in deep-water diving. In Dewar even indicated that the Selig, and John G. Malm, at Ar- contrast, highly polarizable xenon has high THIS ELEMENT new gas could be an allotrope of BROOOHT gonne National Laboratory pre- solubility and is an excellent anesthetic. TO YOU BY nitrogen, N 3 , a suggestion secpared XeF4. Syntheses ofXeF2, The low ionization potentials of the heavAVEC1A onded by Mendeleyev! Within XeF6, XeOF4, Xe0 2 F 2 , Xe0 3 , ier gases also account for their chemistry three years, however, Ramsay and perxenates (XeC^4- salts) In all of the known chemical com(and coworker Morris Travers) were quickly reported from pounds of the noble gases, the noble-gas had also discovered helium, there and elsewhere. Even the atom has a net positive charge. We can take neon, krypton, and xenon. They highly unstable tetrahedral the difluorides as representative. In each, established their monatomic tetroxide Xe0 4 was made (J. L. the noble-gas atom can be viewed as havand unreactive nature. Houston, 1964). A fluoride of ing lost an electron to the two F ligands. krypton, prepared and correct- One way to picture this is to make the pseuWith the advent of the Ruthly identified as KrF 2 (George do halogen Ng+ (transferring the electron erford-Bohr atom (1913), elecC. Pimentel and J. J. Turner, 1963), was to an F to make F~~), the Ng+ combining tron configurations of these unreactive monatomic elements soon came to have a first reported as KrF4 (Aristid V Grosse with the second F to make the classical central role in the emerging electronic theories ofchemical bonding. In their 1916 papers, both G. N. Lewis and W Kossel pointed to the electron configurations of these elements as especially stable. In each theory, the chemical properties of atoms of other elements were tied to the gain or loss of electrons from the configuration of the nearest monatomic gas. So successful were these theories in accounting for a wide range of chemical properties of the elements that the monatomic-gas electron configurations came to be thought of as chemically inviolate. This was fostered by early failures to make compounds of the gases (including an attempt by Henri Moissan to prepare an argon fluoride in 1895). Nevertheless, in his classic 1916 paper, Kossel made an astute observation relevant to the chemical reactivity of these START OF SOMETHING NEW Bartlett's oxidation of xenon by PtF6, shown elements. On the basis of the first ionization po- here, sparked the field of noble-gas chemistry. tentials of the gases, Kossel noted that + xenon was most likely to have the capa- and coworkers, 1963), but no compound electron-pair-bound {Ng-F} (Ng = noble gas). Each difluoride is then represented above Kr(II) has ever been established. Albility of formingfluoridesand oxides. He as a resonance hybrid of the canonical also allowed that a kryptonfluoridemight though the easier ionization of radon leads + be made. Similar predictions were made one to expect the most extensive chem- forms {F"INgF] } and {{FNgJ+F"}. Bond energies in the isoelectronic relatives of the later, byAndreas von Antropoff (1924) and istry for that element, the high instability + NgF species, C1F, BrF, and IF, do not vary of even the most stable isotope has severely by Linus C. Pauling (1932), based on chemical trends in the periodic table. These pre- limited studies of it. L. Stein, ofArgonne, greatly, so it is reasonable+ to assume the dictions led D. M. Ifost (with student A. L. established (in 1962) the existence of a same to hold for the NgF species.
THE NOBLE GASES
T
32
C&EN / SEPTEMBER 8, 2003
HTTP://WWW.CEN-ONLINE.ORG
|
i§
i
Benefit
from
World Leading
p^ltoJoqies $
S
T--
X
*""^
Avecia
•wwjavecia.com
Fiber Optic Oxygon Sensor W e ' v e developed a ruthenium dye-tipped probe that uses fluorescence quenching to measure dissolved or gaseous 0 2 concentration — without consuming o x y g e n . •
Probes do not consume oxygen, allowing for continuous contact with viscous samples such as foods and slurries, pharmaceuticals and biological samples
• Options range from 18-gauge needle probes for penetrating vial septa to robust %" OD probes for process applications •
Unlike electrodes, probes do not require frequent calibration
• Applications include headspace measurement, blood and tissue analysis, and food and beverage QC
For all your photonics needs, visit OceanOptics.com 727.733.2447 • Fax 727.733.3962
$M£$§$ncnce uncovers
In drug discovery, experience counts. The ability to foresee what will work and what won't early, can be the difference between success and failure in a project. Argenta's experience is proven: our scientists have appeared 170 times as inventors on patents and 450 times as authors of publications. Integrating biology and chemistry, from target to candidate, Argenta can add peer quality resources to your team and help you deliver the highest quality results. T o bring precision, quality, value and experience to your drug discovery, contact Argenta.
Targer argenta" dynamic medicinal chemistry and biology™
THE NOBLE GASES AT A GLANCE Name: Noble gases, inert gases. The six noble gases are found in the far right column of the periodic table. Helium's (He) name originates from the Greek helios, meaning sun; neon (Nel from the Greek neos, meaning new. Argon (Ar) derives from the Greek argon, meaning inactive; krypton (Kr) from the Greek kryptos, meaning hidden. Xenon (Xe] comes from the Greek xenos, stranger, while radon's (Rn) name hails from the element radium. Atomic mass: He: 4.00; Ne: 20.18; An 39.95; Kr: 83.80; Xe: 131.29; Rn: (222). History: Helium was discovered on Earth in 1895 by Scottish chemist Sir William Ramsay, though some credit the discovery of helium (in the sun's spectrum) to Janssen and Lockyear in 1868. Ramsay discovered most of the remaining noble gases—argon in 1894 (with Lord Rayleigh) and krypton, neon, and xenon in 1898 (with Morris M. Travers). Radon was discovered in 1898 by Fredrich Ernst Porn. Occurrence: Helium comes from natural gas deposits within Earth and can be isolated from air. Helium has the lowest boiling point of any element—4.2 K. Neon, argon, and krypton are obtained from fractional distillation of liquid air. Radon comes from radium decay. Behavior: These etements were considered to be inert gases until the 1960s, because their oxidation number of zero prevents the noble gases from forming compounds readily. Atl noble gases have the maximum number of electrons possible in their outer shell (two for helium, eight for all others), making them highly stable. Uses: Helium is used in balloons and in deep-sea diving to dilute oxygen that divers breathe. Neon, argon, and krypton are used in lighting. Radon is radioactive and hazardous.
So, in making the difluoride from the constituent atoms via such species, the energy term that changes most from one Ng to another is the ionization potential. In support of this, we note that the heats of atomization (in kilocalories per mole) of XeF 2 (65) and KrF2 (23) differ by almost the same energy as the first ionization potentials, about 44 kcal per mol. On this basis, since the first ionization potential of Ar is 42 kcal per mol higher than 34
C & E N / S E P T E M B E R 8, 2 0 0 3
as Lewis bases, but evidently they cannot that of Kr, it can be supposed that the heat compete with HF. of atomization ofArF2 would be less than Many recent findings, including the first that ofKrF2 by roughly that amount. Since evidence for an argon compound, have the heat of atomization of KrF2 is only 23 come from matrix-isolation studies at the kcal per mol, this implies that ArF2 canUniversity of Helsinki in Finland (Markku not be made. Rasanen and coworkers). These studies Because of the high electronegativity of have established the existence of a large the positively charged noble-gas centers, variety of novel compounds, all stable any ligands for those centers must themup to 4 0 K. Included are H X e O H , selves be electronegative. The ligands must HXeCCH, HKrCN, HKrCCH, and also be small to provide high Coulomb enHArF. The last requires comment, because ergy. The known chemistry is in accord of the nonexistence ofArF2. with this. For compounds that can be prepared and manipulatIn all of these compounds, the ed at ordinary laboratory temvibrational spectroscopic findperatures, ligands are, so far, few ings indicate that the canonical They are, for xenon: oxygen (S. form ({HNg}+Y~) contributes M. Williamson et al., 1963; N. importantly to the binding of the Bartlett, and coworkers, 1969), molecules. The tiny proton is nitrogen (D. D. DesMarteau, highly electronegative, and it 1981) and carbon (D. Naumann; bonds covalently to Ng in these THIS ELEMENT HJ.Frohn,1989);andforkrypmolecules. The proton affinities BROUGHT TO YOU BY ton(GJ. Schrobilgen, 1988-89): of the noble gases are the folSHIN-ETSU oxygen, and nitrogen. In all, the lowing: He, 1.8; Ne, 2.2; Ar, 3.0; nitrogen and carbon ligands, to Kr,>4;andXe,>6eV be effective, require linkage to Attachment of a proton to the other electronegative centers. more polarizable gases therefore The large size of the chlorine ligives significant energy toward gand means that XeCl2 can onbonding. Ofcourse, this requires ly be made and used at cryogenic a small electronegative coligand, temperatures. The high bond of which F has no superior; but energy of 0 2 also leads to high OH, CN, and CCH are also thermodynamic instability of small ligands with high electron THIS ELEMENT all oxides. XeO is only bound affinities. BROUGHT with respect to singlet oxygen, TO YOU BY All of the noble-gas commost likely ID [O]. (E. H. ApSOCMA pounds are easily reduced, and pelman of Argonne used XeF 2 the compounds can effectively in aqueous solution to prepare carry their ligands as radical the first examples of perbrospecies, readily available to more mates, and this could have inreactive elements. For example, volved attack of bromate by KrF2, which is less bound than F 2 XeO as a singlet oxygen carrier.) itself, will oxidize Xe to XeF 6 ! More electronegative cationic Because of its lower effective species such as KrF+ are even nuclear charge, xenon is more more potent (for example, the easily oxidized than krypton. It synthesis of BrF6+, RonaldJ. Gilis also a better Lewis base. Kryplespie, and Schrobilgen, 1974). ton is not oxidized by PtF 6 , and Some of the potential reagents in the solvent anhydrous HF are very fragile and will need to (aHF), forms no complexes with be used at cryogenic temperatransition-metal ions as xenon tures, but the elimination of an does. The HF-solvated Ag2* ion atom of an almost inert gas can simplify (silver has the highest second ionization pothe chemistry tential of any transition metal) oxidizes Xe at ordinary temperatures (Zemva et al., N e i I Ba rt lett is an emeritusprofessor in the de1990) to makeXeF2. With the solvatedAu2+ ion, however (Konrad Seppelt, 2 0 0 1 - 03), partment ofchemistry, University of California, Berkeley, and an emeritus senior scientist in the no oxidation of Xe occurs. Xenon is a sufChemical Sciences Division ofLawrence Berkeficiently good Lewis base, however, to co2+ ley National Laboratory. Bartlett succeeded in ordinate with Au to form a variety of coordination complexes. It similarly com- preparing the first noble-gas compound in 19 62.The Royal Society (London) awarded him plexes with Au3+ and with other metal ions. its Davy Medal in 2002. It is possible that Kr and Ar could also act HTTP://WWW.CEN-ONLINE.ORG
