Question

Which of the following is correct regarding ionic radii?

• A. Ti⁴⁺ < Mn⁺⁷
• B. ₃₅Cl^- < ₃₇Cl^-
• C. K⁺ > Cl^-
• D. P³⁺ > P⁵⁺

Answer 1

The correct option is D

P³⁺ > P⁵⁺

Explanation of correct option:

(D) P³⁺ > P⁵⁺:

• The greater the positive charge on the cation, the lesser the size of the cation due to the more per electron effective nuclear charge.
• Here, both are the cations of P(phosphorus) which has 15 protons , but P³⁺ has electronic configuration of $\left[\mathrm{Ne}\right]3{\mathrm{s}}^{2}3{\mathrm{p}}^0$ and P⁵⁺ has $\left[\mathrm{Ne}\right]3{\mathrm{s}}^{0}3{\mathrm{p}}^0$ , hence the ionic radii of P³⁺ > P⁵⁺. because the former has two electrons more than the latter.
• Hence, the correct order of ionic radii is P³⁺ > P⁵⁺.

Explanation of incorrect option:

(A) Ti⁴⁺ < Mn⁺⁷:

• Ti⁴⁺(titanium ion) and Mn⁺⁷ (manganese ion) have the same number of electrons i.e. 18. So, they are isoelectronic species.
• Atoms or ions that have the same number of electrons are known as isoelectronic species
• The greater the positive charge on the cation, the lesser the size of the cation due to the more per electron effective nuclear charge.
• Here, Ti⁴⁺ has 22 protons and 18 electrons while Mn⁺⁷ has 25 protons and 18 electrons.
• So, Mn⁺⁷ has a greater per electron effective nuclear charge than Ti⁴⁺.
• Hence, Mn⁺⁷ has smaller ionic radii than Ti⁴⁺.

(B) 35Cl^- < 37Cl^-

• They (35 Cl^- and 37 Cl^-) are isotopes of Chlorine.
• Isotopes are two or more types of atoms that have the same atomic number and position in the periodic table, and that differ in nucleon numbers (sum of numbers of protons and neutrons) due to different numbers of neutrons in their nuclei.
• Here, both 35 Cl^- and 37 Cl^- have the same number of protons and electrons, and they have the same position in the periodic table.
• Hence, they both have the same ionic radii.

(C) K⁺ > Cl^-

• K⁺(potassium ion) and Cl^-(chlorine ion) have the same number of electrons ie. 18. So, they are isoelectronic species.
• Atoms or ions that have the same number of electrons are known as isoelectronic species.
• The size of an anion is always greater than the size of a cation in the case of isoelectronic species because anions have less per electron effective nuclear charge and in the anion, the repulsion between electrons is very high. So the size of an anion is larger than a cation.
• Here, K⁺ has 19 protons and 18 electrons while Cl^- has 17 protons and 18 electrons.
• So, Cl^- experiences a lesser per electron effective nuclear charge than K⁺.
• Thus, Cl^- has larger ionic radii than K⁺.

Conclusion: Hence, option (D) is correct.