Legitimate trade, by definition, is not illegitimate.
The differences is that the legitimate child is the legal child while the illegitimate child is the illegal child.
Because they have the outer electron shell full, with 8 electrons; this structure lead to a very low chemical reactivity.
Because they're located on the very right of the periodic table, meaning that they have full outer shells. What atoms "want" in a reaction is to have full outer shells, so noble gases don't react...
For some questions, context is all-important. This is one of those questions.