The Group 18 Elements He, Ne, Ar, Kr, Xe, Rn

The closed-shell electronic structures of the noble gas atoms are extremely stable, as shown by the high ionization enthalpies, especially of the lighter members (Table 14-1). The elements are all low-boiling gases whose physical properties vary systematically with atomic number. The boiling point of helium is the lowest of any known substance. The boiling points and heats of vaporization increase monotonically with increasing atomic number. 

The heats of vaporization are measures of the work that must be done to overcome interatomic attractive forces. Since there are no ordinary electron-pair interactions between noble gas atoms, these weak forces (of the van der Waals or London type) are proportional to the polarizability and inversely proportional to the ionization enthalpies of the atoms they increase therefore as the size and diffuseness of the electron clouds increase. 

The noble gases occur as minor constituents of the atmosphere (Table 14-1). Helium is also found as a component (up to ca. 7%) in certain natural hydrocarbon gases in the United States. This helium undoubtedly originated from decay of radioactive 

Outer Shell Element Configuration Atomic Number First Ionization Enthalpy (kJ mol-1) Normal bp (K) A Hvlp (kJ mol 1) % by Volume in the Atmosphere Promotion Energy (kJ mol 1) nship6 — /w2np5(n+l), 

Liquid Ar, Kr, and Xe are commonly used as solvents for spectroscopic and synthetic studies 1 supercritical Xe is transparent over the range vacuum uv to far ir and is also useful for nmr studies. Examples of spectroscopic studies are those on N,0 species2 such as ONNO, 0NN02, N204, and NO and photochemical reactions of compounds such as Cp Rh(CO)2, Cp Mn(CO)3, or Cp V(CO)43 ir spectra of ZrHt and HfH4 trapped in solid argon have been obtained.4 

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The noble gases are found in group 18 (VIIIA), which according to some older versions of the periodic table is called group 0. These six elements (He, Ne, Ar, Kr, Xe, and Rn) are inert and have a zero oxidation state. They also have full outer valence shells and represent the end of each period of the periodic table. Helium is placed at the beginning of group 18 (VIIIA) because its outer valence shell is completed and it is inert. 

The elements He, Ne, Ar, Kr, Xe, and Rn are called the argonons (or the inert gases, or the noble gases). Their atoms have little tendency to form chemical bonds. The small chemical reactivity of the argonons is attributed to the special stability of groups of 2. 10, 18. 36. 54, and 86 electrons about one atomic nucleus. 

The Group 18 elements are called the noble gases because of their low reactivity. Like a noble man (as opposed to a nobleman) who does not interfere with others, these elements rarely involve themselves in reactions with other atoms. These gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). 

The Group 8A(18) elements are helium (He), the second most abundant element in the universe neon (Ne) argon (Ar) krypton (Kr) xenon (Xe) and radioactive radon (Rn). Only the last three form compounds [Group 8A(18) Family Portrait], The noble gases make up about 1% by volume of the atmosphere, primarily due to the abundance of Ar. 

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