Learning Objective


During the final years of the 19th century, all of the noble gases were isolated for the first time in a very short amount of time.It was not until the 18th century that early works on the composition of air suggested that air contained gases in addition to oxygen, nitrogen, carbon dioxide, and water vapor.During a solar eclipse in 1868, spectral lines of helium, previously unknown on the sun, were discovered during the first discovery of noble gases.A more detailed description of spectroscopy can be found in Chapter 6 "The Structure of Atoms", although actual samples of helium were not obtained until almost 30 years later.John. .William Ramsay and him isolated the new "substance" (not necessarily a new element) from residual nitrogen gas in 1894.Argon (Ar) was named for its inability to decompose or react with anything, resulting in the Greek word argos, which means "lazy." Because its molar mass was 39.9 g/mol, Ramsay proposed that it was a member of a new group of elements located after the halogens and above the alkali metals in the periodic table.Furthermore, he suggested that these elements should have a preferred valence of 0, somewhere between the +1 of alkali metals and the *1 of halogens.


J. W. Strutt (Lord Rayleigh) (1842–1919)

Lord Rayleigh was one of the few members of the British nobility to be recognized as a science genius.Because of his frail health, he was never expected to reach maturity during his youth.Math was his forte at Trinity College, Cambridge, where he excelled. .As a result of leaving the management of the business to his younger brother, Lord Rayleigh was able to devote his time to scientific research.Cambridge University awarded him honorary degrees in science and law.


Sir William Ramsay (1852–1916)

Born and raised in Glasgow, Scotland, Ramsay was expected to become a Calvanist minister.Instead, when he read about the manufacture of gunpowder, he was attracted to chemistry.Dr. Ramsay received his doctorate in organic chemistry from the University of Tübingen in Germany in 1872.In England, he turned first to physical chemistry and then to organic chemistry.


Ramsey collected the first terrestrial sample of helium in 1895. .During an investigation of the radioactivity in the air around the newly discovered radioactive elements radium and polonium in 1900, the German chemist Friedrich Dorn discovered the last noble gas.The element was named radon (Rn), and Ramsay successfully measured the density (and therefore the atomic mass) of radon in 1908.Both Rayleigh and Ramsay received Nobel Prizes for their discoveries of noble gases in 1904.Helium has the lowest boiling point (4.2 K), so it is primarily used as a cryogenic liquid.


Preparation and General Properties of the Group 18 Elements

Unless helium, only fractional distillation of liquid air produces all its noble gases.Helium may be the second most abundant element in the universe (after hydrogen), but because of its low molecular mass and high mean velocity, the helium originally present on Earth's atmosphere was lost long ago.In contrast, natural gas contains relatively high levels of helium (up to 7%), making it the only practical terrestrial source. As a group 18, group 18 elements all have closed-shell electron configurations, such as ns2np6 or 1s2 for He.As is consistent with periodic trends in atomic properties, these elements have high ionization energies that decrease smoothly into the next group down. .Oxidation of noble gases and formation of compounds in positive oxidation states require a powerful oxidant.With F, O, and possibly Cl, xenon and krypton should form covalent compounds with formal oxidation states (+2, +4, +6, and possibly +8), just as the heavier halogens do.These predictions are actually the summation of the chemistry observed for these elements.


Table 22.7 Selected Properties of the Group 18 Elements

df
atomic symbol He Ne Ar Kr Xe Rn
atomic number 2 10 18 36 54 86
atomic mass (amu) 4.00 20.18 39.95 83.80 131.29 222
valence electron configuration* 1s2 2s22p6 3s23p6 4s24p6 5s25p6 6s26p6
triple point/boiling point (°C) —/−269† −249 (at 43 kPa)/−246 −189 (at 69 kPa)/−189 −157/−153 −112 (at 81.6 kPa)/−108 −71/−62
density (g/L) at 25°C 0.16 0.83 1.63 3.43 5.37 9.07
atomic radius (pm) 31 38 71 88 108 120
first ionization energy (kJ/mol) 2372 2081 1521 1351 1170 1037
normal oxidation state(s) 0 0 0 0 (+2) 0 (+2, +4, +6, +8) 0 (+2)
electron affinity (kJ/mol) > 0 > 0 > 0 > 0 > 0 > 0
electronegativity 2.6
product of reaction with O2 none none none none none none
type of oxide acidic
product of reaction with N2 none none none none none none
product of reaction with X2 none none none KrF2 XeF2, XeF4, XeF6 RnF2
product of reaction with H2 none none none none none none

Reactions and Compounds of the Noble Gases

The only compounds noble gases could form were clathrates for many years. .When clathrate is dissolved or melted, the guest molecules (Xe) are immediately released because chemical bonds do not form between the Xe guest molecules and the host molecules (H2O in the case of xenon hydrate).As well as the noble gases, many other species are stable clathrates.An example of this is methane hydrate, which is found naturally at the bottom of the oceans in large quantities.It is estimated that the amount of methane in these deposits could have a major impact on the world's energy needs later this century.


Figure 22.16 The Structure of Xenon Hydrate, a Clathrate

*

For example, Xe or CH4 can occupy the hollow spaces in a lattice of hydrogen-bonded water molecules to produce a structure with a fixed stoichiometry (Xe*5.75H2O in this case).Warming the solid hydrate or decreasing the pressure of the gas causes it to collapse, leading to the evolution of gas and the formation of liquid water.


*

Burning snowballs. Xenon and methane (CH4) are clathrates that form a crystalline structure with water called methane hydrate.When methane is released from the solid, it can be ignited, giving the appearance of burning snow.


.As xenon's ionization energy (1170 kJ/mol) is actually lower than that of oxygen, Bartlett predicted PtF6 could also oxidize xenon.Xenon gas suddenly changed color into orange crystals when it was mixed with PtF6 vapor (Figure 22.17 "The Synthesis of the First Chemical Compound of Xenon").While Bartlett first suggested they were Xe+PtF6*, it has now been widely accepted that this reaction also includes the transfer of a fluorine atom to xenon to produce an XeF+ ion:


Equation 22.46

(s)

Figure 22.17 The Synthesis of the First Chemical Compound of Xenon

*

(a) An apparatus containing platinum hexafluoride, the red vapor at the bottom left, and xenon, the colorless gas in the small tube at the upper right. (b) When the glass seal separating the two gases is broken and the gases are allowed to mix, a bright yellow solid is formed, which is best described as XeF+PtF5−.


Subsequent work showed that xenon reacts directly with fluorine under relatively mild conditions to give XeF2, XeF4, or XeF6, depending on conditions; one such reaction is as follows:


Equation 22.47

Xe(g) + 2F2(g) → XeF4(s)

The ionization energies of helium, neon, and argon are so high (Table 22.7 "Selected Properties of the Group 18 Elements") that no stable compounds of these elements are known. The ionization energies of krypton and xenon are lower but still very high; consequently only highly electronegative elements (F, O, and Cl) can form stable compounds with xenon and krypton without being oxidized themselves. Xenon reacts directly with only two elements: F2 and Cl2. Although XeCl2 and KrF2 can be prepared directly from the elements, they are substantially less stable than the xenon fluorides.


Note the Pattern

The ionization energies of helium, neon, and argon are so high that no stable compounds of these elements are known.


Because halides of the noble gases are powerful oxidants and fluorinating agents, they decompose rapidly after contact with trace amounts of water, and they react violently with organic compounds or other reductants. The xenon fluorides are also Lewis acids; they react with the fluoride ion, the only Lewis base that is not oxidized immediately on contact, to form anionic complexes. For example, reacting cesium fluoride with XeF6 produces CsXeF7, which gives Cs2XeF8 when heated:


Equation 22.48

XeF6(s) + CsF(s) → CsXeF7(s)

Equation 22.49

2 CsXeF 7 (s) → Δ Cs 2 XeF 8 (s)  +  XeF 6 (g)

The XeF82− ion contains eight-coordinate xenon and has the square antiprismatic structure shown here, which is essentially identical to that of the IF8− ion. Cs2XeF8 is surprisingly stable for a polyatomic ion that contains xenon in the +6 oxidation state, decomposing only at temperatures greater than 300°C. Major factors in the stability of Cs2XeF8 are almost certainly the formation of a stable ionic lattice and the high coordination number of xenon, which protects the central atom from attack by other species. (Recall from Section 22.4 "The Elements of Group 16 (The Chalcogens)" that this latter effect is responsible for the extreme stability of SF6.)



For a previously “inert” gas, xenon has a surprisingly high affinity for oxygen, presumably because of π bonding between O and Xe. Consequently, xenon forms an extensive series of oxides and oxoanion salts. For example, hydrolysis of either XeF4 or XeF6 produces XeO3, an explosive white solid:


Equation 22.50

XeF6(aq) + 3H2O(l) → XeO3(aq) + 6HF(aq)

Treating a solution of XeO3 with ozone, a strong oxidant, results in further oxidation of xenon to give either XeO4, a colorless, explosive gas, or the surprisingly stable perxenate ion (XeO64−), both of which contain xenon in its highest possible oxidation state (+8). The chemistry of the xenon halides and oxides is best understood by analogy to the corresponding compounds of iodine. For example, XeO3 is isoelectronic with the iodate ion (IO3−), and XeF82− is isoelectronic with the IF8− ion.


Note the Pattern

Xenon has a high affinity for both fluorine and oxygen.


Because the ionization energy of radon is less than that of xenon, in principle radon should be able to form an even greater variety of chemical compounds than xenon. Unfortunately, however, radon is so radioactive that its chemistry has not been extensively explored.


Example 10

On a virtual planet similar to Earth, at least one isotope of radon is not radioactive. A scientist explored its chemistry and presented her major conclusions in a trailblazing paper on radon compounds, focusing on the kinds of compounds formed and their stoichiometries. Based on periodic trends, how did she summarize the chemistry of radon?

Given: nonradioactive isotope of radon

Asked for: summary of its chemistry

Strategy:

Based on the position of radon in the periodic table and periodic trends in atomic properties, thermodynamics, and kinetics, predict the most likely reactions and compounds of radon.

Solution:

We expect radon to be significantly easier to oxidize than xenon. Based on its position in the periodic table, however, we also expect its bonds to other atoms to be weaker than those formed by xenon. Radon should be more difficult to oxidize to its highest possible oxidation state (+8) than xenon because of the inert-pair effect. Consequently, radon should form an extensive series of fluorides, including RnF2, RnF4, RnF6, and possibly RnF8 (due to its large radius). The ion RnF82− should also exist. We expect radon to form a series of oxides similar to those of xenon, including RnO3 and possibly RnO4. The biggest surprise in radon chemistry is likely to be the existence of stable chlorides, such as RnCl2 and possibly even RnCl4.

Exercise

Predict the stoichiometry of the product formed by reacting XeF6 with a 1:1 stoichiometric amount of KF and propose a reasonable structure for the anion.

Answer: KXeF7; the xenon atom in XeF7− has 16 valence electrons, which according to the valence-shell electron-pair repulsion model could give either a square antiprismatic structure with one fluorine atom missing or a pentagonal bipyramid if the 5s2 electrons behave like an inert pair that does not participate in bonding.


Summary

The noble gases have a closed-shell valence electron configuration. The ionization energies of the noble gases decrease with increasing atomic number. Only highly electronegative elements can form stable compounds with the noble gases in positive oxidation states without being oxidized themselves. Xenon has a high affinity for both fluorine and oxygen, which form stable compounds that contain xenon in even oxidation states up to +8.


Key Takeaways

The noble gases are characterized by their high ionization energies and low electron affinities. Potent oxidants are needed to oxidize the noble gases to form compounds in positive oxidation states.

Conceptual Problems


The chemistry of the noble gases is largely dictated by a balance between two competing properties. What are these properties? How do they affect the reactivity of these elements?


Of the group 18 elements, only krypton, xenon, and radon form stable compounds with other atoms and then only with very electronegative elements. Why?


Give the type of hybrid orbitals used by xenon in each species.

XeF2 XeF4 XeO3 XeOF4 XeO4 XeO64−

Which element is the least metallic—B, Ga, Tl, Pb, Ne, or Ge?