Why were noble gases considered to be inert?

Answer 1

Noble gases have completely filled orbitals / energy levels. They generally have 8 valence electrons (helium has only 2 valence electrons) and have stable electronic configuration. Hence they are chemically inert and generally donot form compounds under normal conditions.

However, under high temperature and pressure, krypton and xenon are known to form compounds with fluorine, oxygen.

Answer 2

Noble gases were considered inert because they have a full outer electron shell, making them stable and unlikely to form chemical bonds with other elements. This stability prevents them from easily reacting or participating in chemical reactions, hence the term "inert."