Group 8A elements

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. 

Group 8A elements (noble gases), such as neon, rarely form covalent bonds because they already have valence-shell octets. 

The Group 8A elements, the noble gases, are characterized by filled s and p valence orbitals (electron configurations of 2s2 for helium and nsznp6 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 19.12. 

The highly reactive group 7A elements are known as halogens, and the extremely unre-active group 8A elements are commonly called the noble gases. 

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