Question

Which of the following trends of ionic radii are correct:

• A. $F^{-}>N{a}^{+}>M{g}^{2+}$
  
• B. $A{l}^{3+}>N{a}^{+}>N^{3-}$
  
• C. $P^{3-}>S^{2-}>C{l}^{-}$
  
• D. $H^{-}>H^{+}>He$

Answer 1

Answer: (A), (C)

$P^{3-}>S^{2-}>C{l}^{-}$


In option (a), (b) and (c) all the species are iso-electronic 10, 10, 18 electrons in each. In iso-electronic species, the ionic radii decreases with increase in atomic number so (a) is correct, (b) is incorrect and (c) is correct.
The size of an anion is larger than that of a neutral atom and that of a cation is smaller than a neutral atom, so
$H^{-}>H^{+}$. \(He\) is a noble gas and hence, its ionic radii cannot be measured as ionic radii are obtained from inter-nuclear distances in ionic compounds. Thus (d) is incorrect.