**Correct Option: C. Iron** ### Detailed Explanation: **What is Ferromagnetism?** Ferromagnetism is a property of certain materials that allows them to become magnetized in the presence of an external magnetic field and retain that magnetization even after the external field is removed. This phenomenon occurs due to the alignment of magnetic moments of atoms within the material. **Why is Iron a Ferromagnet?** 1. **Atomic Structure**: Iron has unpaired electrons in its atomic structure, specifically in the 3d subshell. These unpaired electrons contribute to a net magnetic moment. 2. **Exchange Interaction**: In ferromagnetic materials like iron, there is a strong exchange interaction between neighboring atoms. This interaction causes the magnetic moments of adjacent atoms to align parallel to each other, creating a strong overall magnetic field. 3. **Curie Temperature**: Iron has a Curie temperature (around 770°C) above which it loses its ferromagnetic properties and becomes paramagnetic. Below this temperature, iron can be magnetized and will retain its magnetization after the external field is removed. ### Why the Other Options are Incorrect: **A. Copper** - **Reason**: Copper is a non-magnetic material. It has a full d-subshell (3d10), meaning all its electrons are paired. This pairing cancels out any magnetic moment, preventing copper from exhibiting ferromagnetism. It does not retain magnetization after an external field is applied. **B. Aluminum** - **Reason**: Aluminum is also a non-magnetic material. Similar to copper, it has a full p-subshell configuration, which results in paired electrons that do not contribute to a net magnetic moment. Therefore, aluminum cannot be magnetized in the same way as ferromagnetic materials. **D. Bismuth** - **Reason**: Bismuth is a diamagnetic material, which means it is repelled by magnetic fields. It does not have unpaired electrons that would allow it to exhibit ferromagnetism. Instead, bismuth's electrons are all paired, leading to a very weak and negative magnetic susceptibility. ### Summary of Key Concepts: - **Ferromagnetism**: Ability to retain magnetization after the external field is removed. - **Iron**: Exhibits ferromagnetism due to unpaired electrons and strong exchange interactions. - **Non-Ferromagnetic Materials**: Copper, aluminum, and bismuth do not retain magnetization due to their electron configurations (paired electrons). ### Revision Summary: - Ferromagnetic materials retain magnetization after the external field is removed. - Iron is a classic example of a ferromagnet due to its atomic structure and exchange interactions. - Non-magnetic materials like copper and aluminum do not exhibit ferromagnetism due to paired electrons. - Bismuth is diamagnetic and repelled by magnetic fields, lacking the properties of ferromagnetism.