Argon is the third of the noble gases or inert gases. It is very non-reactive. So much so, that it forms compounds with virtually no other elements. ... It is non-reactive because the shells are full.
Noble gases are the least reactive of all known elements. That's because with eight valence electrons, their outer energy levels are full. The only exception is helium, which has just two electrons. ... Therefore, noble gases are rarely involved in chemical reactions and almost never form compounds with other elements.
The halogen are reactive because of the outermost electrons of its shell, all atom wants to complete the electrons of the outermost shell with eight electrons or two. For the halogens it has 1 electron at it’s outermost shell and it wants to get rid of it that means it loses electrons easily to complete the octet rule. When it loses electrons easily, other atoms reacts or gains the electrons easily. Remember that gaining electrons releases energy.