Which of the following is ferromagnetic

Options

A. `Ni`
B. `Co`
C. `CrO_(2)`
D. All

Answer

To determine which of the given options is ferromagnetic, we need to analyze the electronic configurations of the elements mentioned and count the number of unpaired electrons. Ferromagnetic materials typically have unpaired electrons that contribute to their magnetic properties.

### Step-by-Step Solution:

1. **Identify the Elements**: The question asks us to identify which of the following elements is ferromagnetic. The elements provided are Nickel (Ni), Cobalt (Co), and Chromium (in its +4 oxidation state, Cr^4+).

2. **Determine the Electronic Configuration of Nickel (Ni)**:
- Nickel has an atomic number of 28.
- The electronic configuration is:
\[ \text{Ni: } [\text{Ar}] 4s^2 3d^8 \]
- In the 3d subshell, we have 8 electrons. The distribution is:
- 3d: ↑↓ ↑ ↑ ↑ ↑ (5 single electrons and 3 paired)
- Count the unpaired electrons: **2 unpaired electrons**.

3. **Determine the Electronic Configuration of Cobalt (Co)**:
- Cobalt has an atomic number of 27.
- The electronic configuration is:
\[ \text{Co: } [\text{Ar}] 4s^2 3d^7 \]
- In the 3d subshell, we have 7 electrons. The distribution is:
- 3d: ↑↓ ↑ ↑ ↑ ↑ (5 single electrons and 2 paired)
- Count the unpaired electrons: **3 unpaired electrons**.

4. **Determine the Electronic Configuration of Chromium in +4 Oxidation State (Cr^4+)**:
- Chromium has an atomic number of 24.
- The electronic configuration of neutral chromium is:
\[ \text{Cr: } [\text{Ar}] 4s^2 3d^5 \]
- For Cr^4+, we remove 4 electrons (2 from 4s and 2 from 3d):
- This results in:
\[ \text{Cr}^{4+}: [\text{Ar}] 3d^3 \]
- The distribution in the 3d subshell is:
- 3d: ↑ ↑ ↑ (3 unpaired electrons)
- Count the unpaired electrons: **3 unpaired electrons**.

5. **Conclusion**:
- Nickel has 2 unpaired electrons and is ferromagnetic.
- Cobalt has 3 unpaired electrons and is ferromagnetic.
- Chromium in +4 oxidation state has 3 unpaired electrons and is also ferromagnetic.
- Therefore, all three elements (Ni, Co, and Cr^4+) are ferromagnetic.

### Final Answer:
All the given options (Nickel, Cobalt, and Chromium in +4 oxidation state) are ferromagnetic.