Question

Why Noble gases do not combine with other elements to form molecules?

Answer 1

Using the Bohr description of electron shells, noble gases have full shells. All of the noble gases have full outer shells. The first noble gas is little helium (He), with a shell that is full with only two electrons. The fact that their outer shells are full means they don't need to react with other elements. In fact, they rarely combine with other elements. That non-reactivity is why they are called inert.