JEE Main · 2021 · 24 Feb 2021 · Shift 2 · d and f Block Elements

Across the $3d$ series the atomic mass rises while the metallic radius contracts, so the density climbs steadily from chromium to copper. Zinc breaks the trend: its completely filled $3d^{10}$ shell takes no part in metallic bonding, so the bonding is weak, the metallic radius is large and the lattice is loosely packed. The measured densities are Zn $7.14$, Cr $7.19$, Fe $7.87$, Co $8.90$ and Cu $8.96\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, so the order of increasing density is $\mathrm{Zn}<\mathrm{Cr}<\mathrm{Fe}<\mathrm{Co}<\mathrm{Cu}$.