What is the correct, observed periodic trend for the basic character of the Lanthanide hydroxides, s — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Due to the steady and relentless Lanthanide contraction, the ionic radius of the $Ln^{3+}$ cations strictly decreases as the atomic number increases from $La$ to $Lu$. As the size of the cation decreases, the polarizing power of the cation increases. According to Fajan's rules, this dramatically increases the covalent character of the $Ln-OH$ bond, making it harder to dissociate in water, thereby steadily decreasing the basic strength from $La(OH)_3$ to $Lu(OH)_3$.