In f-block element, due to lanthanide contraction, the size of M3+ ions (i.e. Ce3+,Lu3+, Gd3+ ) decreases and thus, basic strength of their hydroxide also decreases. The order of size of given M3+ ions is Ce3+>Gd3+>Lu3+. Hence, Ce(OH)3 is the most basic hydroxide among all of these. Lanthanide contraction ∝