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:

(D), (A), (C), (B)

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)