Consier the following ions
1. `Ni^(2+)`
2. `Co^(2+)`
3. `Cr^(2+)`
4. `Fe^(3+)`
Atomic number : `Cr=24, Fe=26, Co=27, Ni=28.`
The correct sequence of increasing order of the number of unpaired electrons in these ions is

Options

A. 1, 2, 3, 4
B. 4, 2, 3, 1
C. 1, 3, 2, 4
D. 3, 4, 2, 1

Answer

To determine the correct sequence of increasing order of the number of unpaired electrons in the given ions, we will first find the electronic configurations of each ion and then count the number of unpaired electrons.

### Step 1: Determine the electronic configuration of each ion.

1. **Nickel ion (Ni²⁺)**:
- Atomic number of Ni = 28
- Ground state configuration: \( [Ar] 3d^8 4s^2 \)
- For Ni²⁺, we remove 2 electrons from the 4s orbital:
- Configuration of Ni²⁺: \( [Ar] 3d^8 \)

2. **Cobalt ion (Co²⁺)**:
- Atomic number of Co = 27
- Ground state configuration: \( [Ar] 3d^7 4s^2 \)
- For Co²⁺, we remove 2 electrons from the 4s orbital:
- Configuration of Co²⁺: \( [Ar] 3d^7 \)

3. **Chromium ion (Cr²⁺)**:
- Atomic number of Cr = 24
- Ground state configuration: \( [Ar] 3d^5 4s^1 \)
- For Cr²⁺, we remove 2 electrons (1 from 4s and 1 from 3d):
- Configuration of Cr²⁺: \( [Ar] 3d^4 \)

4. **Iron ion (Fe³⁺)**:
- Atomic number of Fe = 26
- Ground state configuration: \( [Ar] 3d^6 4s^2 \)
- For Fe³⁺, we remove 3 electrons (2 from 4s and 1 from 3d):
- Configuration of Fe³⁺: \( [Ar] 3d^5 \)

### Step 2: Count the number of unpaired electrons in each ion.

1. **Ni²⁺ (3d⁸)**:
- Distribution: ↑↓ ↑↓ ↑↓ ↑ ↑
- Number of unpaired electrons = 2

2. **Co²⁺ (3d⁷)**:
- Distribution: ↑↓ ↑↓ ↑↓ ↑ ↑ ↑
- Number of unpaired electrons = 3

3. **Cr²⁺ (3d⁴)**:
- Distribution: ↑ ↑ ↑ ↑
- Number of unpaired electrons = 4

4. **Fe³⁺ (3d⁵)**:
- Distribution: ↑ ↑ ↑ ↑ ↑
- Number of unpaired electrons = 5

### Step 3: Arrange the ions in increasing order of unpaired electrons.

- Ni²⁺: 2 unpaired electrons
- Co²⁺: 3 unpaired electrons
- Cr²⁺: 4 unpaired electrons
- Fe³⁺: 5 unpaired electrons

### Final Sequence:
The correct sequence of increasing order of the number of unpaired electrons is:
**Ni²⁺ < Co²⁺ < Cr²⁺ < Fe³⁺**