(solved)Question 3.13 of NCERT Class XI Chemistry Chapter 3

$\displaystyle \underline{\underline{\text{size of cation}}}$

Cation is formed by removal of one or more electrons from a parent atom.

Point 1: repulsions between electrons in the same principal shell decrease because there are lesser number of electrons left.

Point 2: the effective nuclear charge experienced by the remaining electrons increases because there are lesser number of electrons to shield one another from the nucleus.

Point 3: the removal of an electron (or electrons) may lead to the removal of a principal shell altogether as in Na+, which is formed by the removal of the 3s1 electron. This is the reason why Na+ is considerably much smaller than Na.

Conclusion 1: the above reasons cause the probability region to decrease and therefore the ion shrinks.

Conclusion 2: among the different cations of the same atom, the ion with the most positive charge is the smallest for the same reasons.

Example 1: Li atom is about 165 pm, whereas the Li+ ion is 75 pm

Example 2: Fe3+ is smaller in size than Fe2+

$\displaystyle \underline{\underline{\text{size of anion}}}$

Anions are formed by addition of 1 or more electrons to a neutral atom.

(i) Mutual repulsions of the electrons, (ii) mutual shielding which leads to a lower Z_eff and (iii) sometimes addition of a new principal shell cause the probability region to expand.

Example: F atom is about 40 pm, whereas the F- anion is 130 pm