Pair of transition metal ions having the same number of unpaired electrons is

Solution

Chemistry ma’am · 3 steps

Write the d-electron count for each ion, then fill the five d orbitals by Hund's rule.

1. Count d electrons. A transition metal ion loses its 4s electrons first. So the d count is the group number minus the charge: Ti is group 4, V 5, Cr 6, Mn 7, Fe 8, Co 9.
2. Fill by Hund's rule. Put one electron in each of the five d orbitals before pairing any. Up to d⁵, unpaired electrons equal the d count. From d⁶ onwards, unpaired electrons equal 10 - n.

   Option	First ion	Second ion	Same?
   (a)	Ti³⁺: d¹, 1 unpaired	Mn²⁺: d⁵, 5 unpaired	No
   (b)	Ti²⁺: d², 2 unpaired	Co²⁺: d⁷, 3 unpaired	No
   (c)	Fe³⁺: d⁵, 5 unpaired	Cr²⁺: d⁴, 4 unpaired	No
   (d)	V²⁺: d³, 3 unpaired	Co²⁺: d⁷, 3 unpaired	Yes

3. So. Only (d) matches. V²⁺ (d³) and Co²⁺ (d⁷) both have 3 unpaired electrons, so the answer is (d).

The idea to keep. For a free ion, dⁿ and d¹⁰⁻ⁿ have the same number of unpaired electrons. So d³ pairs with d⁷, d² with d⁸, and d⁴ with d⁶.

Answer from the NTA final answer key. Solution by PrepWiser.