Question

What is the trend observed in the ionic size of the following isoelectronic species? Explain.

i. Na⁺, Mg²⁺, Al³⁺ and Si⁴⁺

ii. O^2-, F^-, Na⁺ and Mg²⁺

Answer 1

i. Na⁺, Mg²⁺, Al³⁺ and Si⁴⁺

a. Among the given ions, the nuclear charge varies but the number of electrons remains the same and therefore, these are isoelectronic species.

b. The radii of isoelectronic species vary according to actual nuclear charge. Larger nuclear charge exerts greater attraction on the electrons and thus, the radius of that isoelectronic species becomes smaller.

c. The nuclear charge increases in the order Na⁺ < Mg²⁺ < Al³⁺ < Si⁴⁺ and thus, the ionic size decreases in the order Na⁺ > Mg²⁺ > Al³⁺ > Si⁴⁺.

ii. O^2-, F^-, Na⁺ and Mg²⁺

a. Among the given ions, the nuclear charge varies but the number of electrons remains the same and therefore, these are isoelectronic species. 

b. The radii of isoelectronic species vary according to actual nuclear charge. Larger nuclear charge exerts greater attraction on the electrons and thus, the radius of that isoelectronic species becomes smaller.

c. The nuclear charge increases in the order O^2- < F^- < Na⁺ < Mg²⁺ and thus, the ionic size decreases in the order O^2- > F^- > Na⁺ > Mg²⁺.