JEE Main 2021 · 24 February · evening · Single correct · +4 / −1
What is the correct order of the following elements with respect to their density?
- (a) Cr < Zn < Co < Cu < Fe
- (b) Cr < Fe < Co < Cu < Zn
- (c) Zn < Cr < Fe < Co < Cu
- (d) Zn < Cu < Co < Fe < Cr
Answer and solution
Answer (c) Zn < Cr < Fe < Co < Cu
Solution
Chemistry ma’am · 3 steps
Density climbs across the 3d row up to copper, then zinc drops all the way to the bottom.
- The general trend. Going left to right across the 3d series, atomic mass increases while atomic radius shrinks or stays about the same. More mass packed into a smaller volume means density rises from Cr to Fe to Co to Cu.
- Why zinc breaks it. Zinc has a completely filled 3d¹⁰ shell, so its d electrons do not take part in metallic bonding. The bonding is weak, the atom is larger, and the atoms pack less tightly. Its density falls to about 7.14 g cm⁻³, just below chromium.
Element Configuration Density (g cm⁻³) Zn 3d¹⁰4s² 7.14 Cr 3d⁵4s¹ 7.19 Fe 3d⁶4s² 7.87 Co 3d⁷4s² 8.90 Cu 3d¹⁰4s¹ 8.96 - So. Arranging from lowest to highest gives Zn < Cr < Fe < Co < Cu, which is option (c).
The idea to keep. In the 3d series, density rises with atomic number up to copper. Zinc is the exception: its full d¹⁰ shell gives weak metallic bonding, so it is the least dense of the group.
Answer from the NTA final answer key. Solution by PrepWiser.