Why were noble gases said to have no electronegativity?
Noble gases do have an electronegativity. Pauling just didn't have them at first because one thing he needed was homonuclear bond energies.
Chemists have been able to calculate their electronegativities as
The calculated electronegativity of
Here's a Periodic Table that includes the electronegativities of the noble gases.
(Adapted from Inorganic Chemistry, Miessler et al., 5th ed., pg. 58)