Oxidation States and Lanthanoid Contraction — Prelims Strategy

Prelims Strategy

To excel in NEET questions on oxidation states and lanthanoid contraction, a multi-pronged strategy is recommended. Firstly, thoroughly understand the general electronic configuration of lanthanoids and the order of electron removal to form ions.

Memorize the common +3 oxidation state and the specific elements (Ce, Eu, Yb, Sm, Tb) that exhibit +2 or +4 states, along with the reason (stable f$^0$, f$^7$, f$^{14}$ configurations). For lanthanoid contraction, focus on its definition, the primary cause (poor shielding of 4f electrons), and its three major consequences: similar radii of 4d and 5d elements (e.

g., Zr/Hf), increased density of 5d elements, and decreasing basicity of hydroxides. Practice identifying strong oxidizing/reducing agents based on their non-+3 oxidation states. Pay close attention to trap options that might confuse the cause with the effect, or misstate the trends in basicity or density.

Use mnemonics for exceptions if helpful. Regular revision of these specific facts and their underlying reasons will be key.