Which of the following is ferromagnetic ?

Options

A. Ni
B. Co
C. `CrO_2`
D. All

Answer

To determine which of the given substances is ferromagnetic, we need to analyze each option based on the properties of ferromagnetism.

### Step-by-Step Solution:

1. **Understanding Ferromagnetism**:
- Ferromagnetic materials are those that can be magnetized; they retain their magnetic properties even after the external magnetic field is removed. This is due to the presence of unpaired electrons that create a permanent magnetic dipole.

2. **Analyzing Each Substance**:
- **Nickel (Ni)**:
- Atomic Number: 28
- Electronic Configuration: [Ar] 3d^8 4s^2
- Nickel has unpaired electrons in the 3d orbital, which contribute to its ferromagnetic properties.

- **Cobalt (Co)**:
- Atomic Number: 27
- Electronic Configuration: [Ar] 3d^7 4s^2
- Cobalt also has unpaired electrons in the 3d orbital, making it ferromagnetic as well.

- **Chromium Dioxide (CrO2)**:
- Chromium in CrO2 is in the +4 oxidation state.
- Atomic Number of Chromium: 24
- Electronic Configuration: [Ar] 3d^5 4s^1
- In CrO2, the chromium ion has unpaired electrons, which gives it ferromagnetic properties.

- **Aluminium (Al)**:
- Atomic Number: 13
- Electronic Configuration: [Ne] 3s^2 3p^1
- Aluminium does not have unpaired electrons in a way that would allow it to exhibit ferromagnetism; it is not ferromagnetic.

3. **Conclusion**:
- The substances that exhibit ferromagnetic properties from the options given are Nickel, Cobalt, and Chromium Dioxide. Aluminium does not exhibit ferromagnetic properties.

### Final Answer:
- Nickel, Cobalt, and Chromium Dioxide are ferromagnetic substances.