Q.The metallic radii of some transition elements are given below : Element | Iron | Cobalt | Nickel | Copper
Metallic radii/pm | 126 | 125 | 125 | 128 An element that has the highest density from the above is .................. .

Unlike a normal period, where atomic radius falls steadily left to right, the 3d series shows the expected decrease only from Sc to V; thereafter, from V through Cu, the radius stays nearly constant. This is because two opposing effects roughly cancel: the added 3d electrons only partially shield the rising nuclear charge (raising effective nuclear charge slightly), while those same 3d electrons strongly repel the 4s electrons (pushing the atom's boundary outward) -- the two forces balance across most of the series. At zinc, the d¹⁰ sub-shell's inter-electron repulsion outweighs the effective nuclear charge, so the orbitals slightly expand and atomic radius ticks back up. …