The behaviour of the lighter members approaches that of an ideal gas at normal temperatures with the heavier members increasing polarizability and dispersion forces lead to easier liquefaction under pressure. Being monatomic the noble gases are spherically symmetrical and have very weak interatomic interactions and consequent low enthalpies of vaporization. The elements thus represent the termination of a period and have closed-shell configuration and associated high ionization energies (He 2370 to Rn 1040 kJ mol −1) and lack of chemical reactivity.
The configurations of the others terminate in ns 2 np 6 and all inner shells are fully occupied. The electron configuration of helium is 1 s 2. A group of monatomic gaseous elements forming group 18 (formerly group 0) of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).
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