JEE Main 2025 · 29 January · morning · Single correct · +4 / −1
The correct option with order of melting points of the pairs (Mn, Fe), (Tc, Ru) and (Re, Os) is :
- (a) Fe < Mn, Ru < Tc and Re < Os
- (b) Fe < Mn, Ru < Tc and Os < Re
- (c) Mn < Fe, Tc < Ru and Re < Os
- (d) Mn < Fe, Tc < Ru and Os < Re
Answer and solution
Answer (d) Mn < Fe, Tc < Ru and Os < Re
Solution
Chemistry ma’am · 3 steps
Mn and Tc dip below their neighbours, but in the 5d row Re stays higher than Os.
- The general trend. Melting points rise across a transition series and peak near the middle (group 6), where the most d electrons take part in metallic bonding. After that, electrons start to pair up and the melting point falls.
- Group 7 dip. Mn (3d⁵ 4s²) has a stable half-filled d subshell. Its d electrons are held tightly and bond weakly, so Mn melts lower than Fe. Tc in the 4d row also melts lower than Ru. In the 5d row the orbitals are larger and bond strongly, so the dip disappears. Re stays very high, second only to W, and Os comes after it.
Pair Melting points (approx.) Order Mn, Fe 1519 K, 1811 K Mn < Fe Tc, Ru 2430 K, 2607 K Tc < Ru Re, Os 3459 K, 3306 K Os < Re - So. The correct order is Mn < Fe, Tc < Ru and Os < Re, which is option (d).
The idea to keep. Melting points peak around group 6. Mn is the famous low point of the 3d row. Do not copy one row's pattern onto the 5d row, because there Re outranks Os.
Answer from the NTA final answer key. Solution by PrepWiser.