Question

Arrange the ions Se^2-, Cl^-, and S^2- in order of increasing ionic radius.

Please explain your answer, Thank You!

Answer 1

The order of increasing ionic radius is:

Cl^- < S^2- < Se^2-

The Cl^- means that Cl atom has gained one electron, S^2- means the S has gained 2 electrons, and similarly Se^2- has gained 2 electrons.

The atomic radius decreases as you move from left to right across a period because of the addition of more and more positively charged protons enter into the nucleus thereby pulling the electrons closer to the nucleus. if you add electrons to the atom but not the protons, the positive charge seen by more electrons is reduced and hence there is slight shift from the nucleus thereby increasing its radius and hence S^2- higher radius the Cl^-. Se^2- is below in the group along with S and hence has a larger size as compared to sulfide ion.