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)

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).

the maximum mass of nh3 is nh3 in grams