The Group 8A Elements

The name for OF2 is oxygen difluoride rather than difluorine oxide because fluorine has a higher electronegativity than oxygen and thus is named as the anion. 

Fluorine forms only one oxyacid, hypofluorous acid (HOF), but it forms at least two oxides. When fluorine gas is bubbled into a dilute solution of sodium hydroxide, the compound oxygen difluoride (OF2) is formed  

The Group 8A elements, the noble gases, are characterized by filled s and p valence orbitals (electron configurations of 2s for helium and ns np for the others). Because of their completed valence shells, these elements are very un-reactive. In fact, no noble gas compounds were known 50 years ago. Selected properties of the Group 8A elements are summarized in Table 18.22. 

Helium was identified by its characteristic emission spectrnm as a component of the sun before it was found on earth. The major sources of helium on earth are natural gas deposits, where helium was formed from the a-particle decay of radioactive elements. The a particle is a helium nucleus that can easily pick np electrons from the environment to form a helium atom. Although helium forms no componnds, it is an important substance that is used as a coolant, as a pressnrizing gas for rocket fuels, as a diluent in the gases used for deep-sea diving and spaceship atmospheres, and as the gas in lighter-than-air airships (blimps). 

Like helinm, neon forms no compounds, but it is a very useful element. For example, neon is widely used in luminescent lighting (neon signs). Argon, which recently has been shown to form chemical bonds under special circumstances, is used to provide the noncorrosive atmosphere in incandescent light bulbs, which prolongs the life of the tungsten filament. 

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Element Melting Point (°C) Boiling Point fC) Atmospheric Abundance (% by volume) Examples of Compounds 

Helium was identified by its characteristic emission spectrum as a component of the sun before it was found on earth. The major sources of helium on earth are natural gas deposits, where helium was formed from the a-particle decay of radioactive elements. The a particle is a helium nucleus that can 

Neon sign maker and artist Jess Baird shows off a few of the items he has made in his Weatherford, Texas, shop. 

Krypton and xenon have been observed to form many stable chanical compounds. The flrst of these was prepared in 1962 by Neil Bartlett (1932-2(X)8), an English chonist who made an ionic compound that he thought had the formula XePtFe. Subsequent studies indicated that the compound might be better represented as XeFPtFe and contains the XeF and PtF ions. 

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Neon has eight valence electrons and all of them are paired, hence the valence orbitals of neon are completely filled. Therefore neon is very unreactive and does not bond with any other element. Similarly, the group 8A elements (noble gases) helium and argon are very unreactive. However, krypton and xenon may form bonds under certain conditions. 

FIGURE 5.1 Agraph of atomic radius in picometers (pm) versus atomic number shows a rise-and-fall pattern of periodicity. The maxima occur for atoms of group 1A elements (Li, Na, K, Rb, Cs, Fr) the minima occur for atoms of the group 7A elements. Accurate data are not available for the group 8A elements. 

The success of these and other predictions convinced chemists of the usefulness of Mendeleev s periodic table and led to its wide acceptance. Even Mendeleev made some mistakes, though. He was completely unaware of the existence of the group 8A elements—He, Ne, Ar, Kr, Xe, and Rn—because none were known at the time. All are colorless, odorless gases with little or no chemical reactivity, and none were discovered until 1894, when argon was first isolated. 

Though sometimes referred to as "rare gases" or "inert gases," these older names are not really accurate because the group 8A elements are neither rare nor completely inert. Argon, for instance, makes up nearly 1% by volume of dry air, and there are several dozen known compounds of krypton and xenon, although none occur naturally. Some properties of the noble gases are listed in Table 6.8. 

The s electron configuration of the group 8A elements (noble gases) is exceptionally stable. 

The group 8A elements, known as the noble gases, are all nonmetals that are gases at room temperature. They are all monatomic (that is, they consist of single atoms rather than molecules). Some physical properties of the noble-gas elements are listed in V TABLE 7.8. The high radioactivity of radon (Rn, atomic number 86) has limited the study of its reaction chemistry and some of its properties. 

The elements of group 8A are chemically unreactive. Indeed, most of our references to these elements have been in relation to their physical properties, as when we discussed intermolecular forces. — (Section 11.2) The relative inertness of these elements is due to the presence of a completed octet of valence-sheU electrons (except He, which only has a filled Is shell). The stability of such an arrangement is reflected in the high ionization energies of the group 8A elements. (Section 7.4)... 

The group 8A elements are aU gases at room temperature. They are components of Earth s atmosphere, except for radon, which exists only as a short-lived radioisotope. 

For the noble gases (the Group 8A elements), 2He, foNe, igAr, fgKr, and i Xe, (a) determine the number of protons and neutrons in the nucleus of each atom, and (b) determine the ratio of neutrons to protons in the nucleus of each atom. Describe any general trend... 

Figure 5.1 Elements that exist as gases at 25°C and 1 atm. The noble gases (the Group 8A elements) are monatomic species the other elements exist as diatomic molecules. Ozone (Os) Is also a gas.

Finally, the last member of the noble gases, radon, was discovered by the German chemist Frederick Dom in 1900. A radioactive element and the heaviest elemental gas known, radon s discovery not only completed the Group 8A elements, but also advanced our understanding about the nature of radioactive decay and transmutation of elements. 

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