Which one of the following does not correctly represent the correct order of the property indicated against it?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(Ti \lt V \lt Cr \lt Mn\): increasing melting point
Transition metals have high melting points because many electrons, both (n−1)d and ns, take part in metallic bonding. Across a series the melting point rises to a maximum at about d5 and then falls, but manganese is an anomaly: its half-filled 3d5 subshell holds its d electrons firmly, so fewer take part in bonding and Mn has a much lower melting point than its neighbours.
Melting points: Ti 1668 °C, V 1910 °C, Cr 1907 °C, Mn 1246 °C
Mn melts lowest of the four, not highest, so the order Ti < V < Cr < Mn for melting point is not correct.
Option (1) \(Ti \lt V \lt Cr \lt Mn\): increasing number of oxidation state — A correct order. The number of oxidation states rises up to Mn: Ti +2 to +4, V +2 to +5, Cr +2 to +6, Mn +2 to +7.
Option (2) \(Ti^{3+} \lt V^{3+} \lt Cr^{3+} \lt Mn^{3+}\): increasing magnetic moment — A correct order. 3d1, 3d2, 3d3, 3d4: 1, 2, 3 and 4 unpaired electrons, so μ rises (1.73, 2.83, 3.87, 4.90 BM).
Option (3) \(Ti \lt V \lt Cr \lt Mn\): increasing melting point — Not the correct order. Mn has the lowest melting point of the four (half-filled 3d5).
Option (4) \(Ti \lt V \lt Mn \lt Cr\): increasing second ionisation enthalpy — A correct order. Ti 1310, V 1414, Mn 1509, Cr 1592 kJ mol−1; \( \ce{Cr^{+}} \) (3d5) loses an electron from a stable half-filled subshell.
NCERT (2026-27 edition):
Physical Properties – “… The transition metals (with the exception of Zn, Cd and Hg) are very hard and have low volatility. Their melting and boiling points are high. Fig. 4.1 depicts the melting points of transition metals belonging to 3d, 4d and 5d series. The high melting points of these metals are attributed to the involvement of greater number of electrons from (n-1)d in addition to the ns electrons in the interatomic metallic bonding. In any row the melting points of these metals rise to a maximum at d5 except for anomalous values of Mn and Tc and fall regularly as the atomic number increases.”
