From titanium (Z = 22) to copper (Z = 29), the density of the metals shows a significant increase. Which of the following is responsible for this increase?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The decrease in metallic radius coupled with the increase in atomic mass
Density is mass per unit volume. Along the 3d series each new electron enters an inner d orbital, which does not shield the outer electrons well, so the metallic radius decreases a little. At the same time the atomic mass increases. More mass packed into a smaller atom gives a higher density.
Why the other options are incorrect
Option (1) The lanthanoid contraction — Incorrect.
The lanthanoid contraction comes from the filling of the 4f orbitals.
It affects the third (5d) series, making its radii almost the same as those of the second series; it has no part in the 3d series from Ti to Cu.
Option (2) The increase in metallic radius coupled with the increase in atomic mass — Incorrect. The radius does not increase along the series; it decreases. A larger radius would spread the mass over a larger volume and lower the density.
Option (3) The increase in the number of unpaired electrons along the series — Incorrect.
The number of unpaired electrons does not rise steadily; it is highest in the middle of the series (Cr, Mn) and falls towards Cu.
It cannot account for a density that keeps rising up to copper.
NCERT (2026-27 edition):
4.3.2 Variation in Atomic and Ionic Sizes of Transition Metals – “The decrease in metallic radius coupled with increase in atomic mass results in a general increase in the density of these elements.”