Question

Which has the highest ionic radii among given ions?
$C{e}^{3+},L{a}^{3+},P{m}^{3+}$ and $Y{b}^{3+}$

• A. $Y{b}^{3+}$
• B. $C{e}^{3+}$
• C. $L{a}^{3+}$
• D. $P{m}^{3+}$

Answer 1

The correct option is C $L{a}^{3+}$

Electronic configuration of ions are:

1. $Y{b}^{3+}:Xe4f^{13}$

2.$C{e}^{3+}:Xe4f^1$

3.$L{a}^{3+}:Xe4f^0$

4.$P{m}^{3+}:Xe4f^4$

Since, the least shielding tendency of f-orbital electron, effective nuclear charge increases and therefore size decreases along the period. Therefore $L{a}^{3+}$ has highest ionic radii.

This phenomenon is called lanthanoid contraction.