Question

 Mg²⁺ ions is smaller than O^2- ion although both are iso-electronic. Explain.

Answer 1

Mg²⁺ ion is smaller than O^2- ion though both are iso-electronic. The nuclear charge in Mg²⁺ is + 12 and O^2- is + 8, so with the increase in nuclear charge the size decreases and, hence O^2- > Mg²⁺.