Why are the noble gases (group 18) inert?

The noble gases are relatively unreactive because they have a stable octet of valence electrons.

Thus, they do not tend to undergo reactions in which they will gain or lose valence electrons,

However, only He, Ne, and Ar are inert. Kr and Xe combine with other highly reactive elements to form stable compounds.

neutrons are in the middle of the atom and the electrons, which are much smaller.