Variation in Atomic and Ionic Sizes — Prelims Strategy

Prelims Strategy

To excel in NEET questions on atomic and ionic sizes of transition elements, a multi-pronged strategy is essential. Firstly, thoroughly understand the general trends: initial decrease, then near constancy, and slight increase across a period (e.

g., 3d series). Memorize the factors causing these trends: increasing $Z_{eff}$, poor shielding by d-electrons, and electron-electron repulsion. Secondly, master the concept of lanthanoid contraction.

Understand its cause (poor shielding of 4f electrons) and all its major consequences (similar sizes of 4d and 5d elements, similar chemical properties, higher densities of 5d elements, higher ionization enthalpies for 5d elements).

This is a high-yield area. Thirdly, practice comparing ionic radii, remembering that cations are smaller than their parent atoms, and for the same element, higher positive charge means smaller ionic radius.

When comparing ions across a series, remember the general decrease due to increasing $Z_{eff}$. Use mnemonics or flashcards to recall specific pairs affected by lanthanoid contraction (e.g., Zr/Hf, Nb/Ta).

Pay close attention to the wording of questions, especially 'increasing' vs. 'decreasing' order. Avoid common traps like assuming a monotonic decrease across a series or misapplying lanthanoid contraction to 3d/4d pairs.