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Atomic radius decreases across a period (increasing Zeff, same shell) and increases down a group (new shell added). Three types: covalent < metallic < van der Waals radius.

In d-block, radius decreases slowly across the series (d-electrons provide moderate shielding). At the end (Cu, Zn), slight expansion occurs from $d^{10}$ electron-electron repulsion.

Lanthanoid contraction: gradual size decrease across lanthanoids due to poor shielding by 4f electrons. Consequence: 5d elements (Hf, Ta, W) have virtually the same radii as 4d elements (Zr, Nb, Mo).

Ionic radii: Cations < parent atom (fewer electrons, same nuclear charge → contraction). Anions > parent atom (more electrons → repulsion → expansion). For isoelectronic species (same electron count), more protons = smaller radius: $O^{2-}$ > $F^-$ > Ne > $Na^+$ > $Mg^{2+}$ > $Al^{3+}$ (all 10 electrons).

This isoelectronic series question is one of the most frequently tested concepts in JEE. The key is: among species with the same number of electrons, nuclear charge is the only variable determining size.