Atomic Radius and Ionic Radius — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on atomic and ionic radii, a systematic approach is essential. First, thoroughly understand the definitions of covalent, metallic, and van der Waals radii, and their relative magnitudes.

Remember that van der Waals radius is always the largest, followed by metallic, then covalent for the same element. Second, internalize the fundamental factors influencing size: nuclear charge (pulls electrons in), number of shells (increases size), and shielding effect (reduces effective nuclear charge).

Across a period: — Atomic radius decreases (due to increasing effective nuclear charge).
Down a group: — Atomic radius increases (due to addition of new shells).

For ionic radii, remember the key rules: Cations are smaller than their parent atoms, and anions are larger. For isoelectronic species (same number of electrons), the size is inversely proportional to the nuclear charge (more protons = smaller size).

When solving MCQs, always identify if the species are in the same period, same group, or are isoelectronic. For numerical problems (though rare for this topic), ensure correct units (picometers, angstroms).

Pay close attention to trap options that reverse trends or misapply rules for isoelectronic species. Practice comparing various combinations of atoms and ions to build confidence. Drawing small diagrams of electron shells can sometimes help visualize the changes in size.