Question

name the gas to be filled in discharge tube to obtain the lightest and smallest positive ion

Answer 1

its hydrogen

the nucleus of hydrogen contains only a single proton and no neutrons,

so upon ionization i.e, removal of the single electron from H-atom, we get +vely charged H-nucleus  which has 1 unit of +ve charge and mass of 1 .67×10⁻²⁴ g

the lightest and smallest possible positive ion

Answer 2

your answer ↓

The gas to be filled in discharge tube to obtain the lightest and smallest positive ion is HYDROGEN because it contains only single proton with no neutron in its nucleus