Physical properties of Group 18 elemnts

    Some physical properties of the group 18 elements are listed in Table 1.1.

Table 1.1 Some physical properties of the group 18 elements (noble gases).

    Of particular significance is the fact that the noble gases have the highest ionization energies of the elements in their respective periods (Figure 1.15), but there is a decrease in values on descending the group. The extremely low values of Δand ∆_vapH^o correspond to the weak van der Waals interactions between the atoms, and the increase in values of ∆_vapH^o down the group is due to increased interatomic interactions as atomic size and polarizability increase.  The properties of He deserve special note; it can diffuse through rubber and most glasses. Below 2.18 K, ordinary liquid ⁴He (but not ³He) is transformed into liquid He(II) which has the remarkable properties of a thermal conductivity 600 times that of copper, and a viscosity approaching zero; it forms films only a few hundred atoms thick which flow up and over the side of the containing vessel.