The element which does not form a monatomic ion is Argon
Explanation
Argon is a group 18 ( noble gas) element.
All noble gas element are inert ( they are non reactive)
Since argon is inert, it does not form a monatomic ion since it has 8 valence electron in it's outermost energy level. ( Argon outermost energy level is fully occupied with electrons)
