Question

Mg²⁺ , Ar, Br , Ca²⁺

arrange in decreasing order of radii

Answer 1

In the given above elements Calcium is having four shells containing electrons. But Ca2+ will have only three shells because the valence electrons are removed.
Bromine without charge is having four shells with electrons.
Argon is having three shells with electrons
Magnesium will be having three shells. But Mg2+ loses two electrons. So it will have only two shells (2,8)
So the correct order will be
Mg2+ (two shell with posite charge)< Ca2+ (Three shell with positive charge)<Ar (Three shells without charge)<Br (four shells)
Here the cation will be having less charge than neutral atom due to more number of protons.