Which of the following elements does not form a monatomic ion?

argon

chlorine

sodium

oxygen

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)

c is having 4 valence electrons and is bonded to three hydrogens and one sulphur atom, therefore, the number of bond pair is equal to four. as all the valence electrons are used in forming the bonds so, there is no lone pair left on carbon.

s is having 6 valence electrons and is bonded to one other sulfur atom and one carbon atom, therefore, the number of bond pair is equal to 2. as only two valence electrons are used in forming the bonds so, four electrons are left. therefore, two lone pairs are present on s.