Arrange the following transition metal ions in the increasing order of unpaired electrons.
(A) $Cu^{2+}$
(B) $Cr^{2+}$
(C) $Ni^{2+}$
(D) $Sc^{3+}$
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (D), (A), (C), (B)
Let's find the number of unpaired electrons in each ion.
Step 1: Determine electron configurations (3d and 4s)
A. Cu²⁺:
- Cu: [Ar] 3d¹⁰ 4s¹
- Cu²⁺: remove 2 electrons → [Ar] 3d⁹
- Unpaired electrons = 1
B. Cr²⁺:
- Cr: [Ar] 3d⁵ 4s¹
- Cr²⁺: remove 2 electrons → [Ar] 3d⁴
- Unpaired electrons = 4
C. Ni²⁺:
- Ni: [Ar] 3d⁸ 4s²
- Ni²⁺: remove 2 electrons → [Ar] 3d⁸
- Unpaired electrons = 2
D. Sc³⁺:
- Sc: [Ar] 3d¹ 4s²
- Sc³⁺: remove 3 electrons → [Ar] 3d⁰
- Unpaired electrons = 0
Step 2: Arrange in increasing order of unpaired electrons
$\text{Sc³⁺ (0)} < \text{Cu²⁺ (1)} < \text{Ni²⁺ (2)} < \text{Cr²⁺ (4)}$
(D)< (A)< (C)<(B)