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 identify the number of unpaired electrons.

### Step 1: Determine the electronic configurations of the ions

1. **Nickel ion (Ni²⁺)**:
- Atomic number of Ni = 28
- Ground state configuration: \( [Ar] \, 3d^8 \, 4s^2 \)
- For Ni²⁺, we remove 2 electrons (from 4s first):
- Configuration: \( [Ar] \, 3d^8 \)
- Unpaired electrons: 2 (since 3d subshell can hold a maximum of 10 electrons, and 8 means there are 2 unpaired)

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

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: \( [Ar] \, 3d^4 \)
- Unpaired electrons: 4 (3d subshell has 4 electrons, which gives 4 unpaired)

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: \( [Ar] \, 3d^5 \)
- Unpaired electrons: 5 (3d subshell has 5 electrons, which gives 5 unpaired)

### Step 2: Count the unpaired electrons

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

### Step 3: Arrange in increasing order of unpaired electrons

Now we can arrange the ions based on the number of unpaired electrons:

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

### Final Answer

The correct sequence of increasing order of the number of unpaired electrons in these ions is:
**Ni²⁺ < Co²⁺ < Cr²⁺ < Fe³⁺**