Cl- < Se3+ is the decreasing order of ionic radii of the two ions.
Explanation:
Atomic number of Se= 34 Se3+ will have 31 electrons (+ sign indicates loss of electron)
Atomic number of chlorine is 17, Cl- will have 18 electrons (- sign shows gain of electrons)
Number of electrons in their valence shell:
Se3+ = 2,8,18,3
Cl- = 2,8,8
The Se3+ atoms have K.L.M.N shells while Cl- K,L,M shells
The size of the ions is based on the effective nuclear charge on the ion. The loss in electron causes small cation and gain in electron causes increase in anion size.
The number of occupied shells more the number more is the ionic radii.
Se3+ will have large ionic radii as Se+ has four filled shells.
Cl- < Se3+ (decreasing order of ionic radii)
