Why are the elements of Group 18 known as noble gases ?

Explanation

The elements present in Group 18 react with a few elements only under certain conditions. It is due to the following facts
(i) The atoms of noble gases have stable closed shell electronic configurations `(ns^2 np^6)`.
(ii) Exceptionally high ionisation energies.
(iii) Positive electron gain enthalpies i.e., energy is needed when an atom of a noble gas takes up an electron.
As all these elements are gases under normal conditions of temperature and pressure, and react with few elements only under certain conditions, therefore they are known as noble gases.