2003 AIPMT Classification of Elements and Periodicity in PropertiesIonic Radii of Isoelectronic Species Easy Question 5 of 5 in Ionic Radii of Isoelectronic Species

The ions $O^{2-}$, $F^-$, $Na^+$, $Mg^{2+}$ and $Al^{3+}$ are isoelectronic. Their ionic radii show :

A A significant increase from $O^{2-}$ to $Al^{3+}$
B A significant decrease from $O^{2-}$ to $Al^{3+}$
C An increase from $O^{2-}$ to $F^-$ and then decrease from $Na^+$ to $Al^{3+}$
D A decrease from $O^{2-}$ to $F^-$ and then increase from $Na^+$ to $Al^{3+}$

The answer is B.

Solution

All five ions have 10 electrons (the neon configuration $1s^2\,2s^2\,2p^6$).
Their nuclear charges are: O = 8, F = 9, Na = 11, Mg = 12, Al = 13.
For the same number of electrons, a larger nuclear charge pulls the electron cloud more strongly, so the ion becomes smaller.
Ionic radii (pm): $O^{2-}$ 140, $F^-$ 133, $Na^+$ 102, $Mg^{2+}$ 72, $Al^{3+}$ 54.
So the ionic radii show a significant decrease from $O^{2-}$ to $Al^{3+}$.

Concept Radius decreases with nuclear charge

Ionic Radii of Isoelectronic Species in past papers

5 questions from this chapter have appeared across 5 exam years.