### 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 to retain that magnetization even after the external field is removed. This is due to the alignment of magnetic moments (tiny magnetic fields produced by the electrons) in the material. **Why is Iron a Ferromagnetic Material?** 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 phenomenon called exchange interaction, which causes the magnetic moments of neighboring atoms to align parallel to each other. This alignment leads to 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 easily magnetized and will retain its magnetization when the external field is removed. **Magnetization Process**: When iron is exposed to a magnetic field, the magnetic domains (regions where the magnetic moments are aligned) within the iron align with the field. Once the external field is removed, many of these domains remain aligned, resulting in a permanent magnet. #### Why the Other Options are Incorrect: - **A. Copper**: - 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 ferromagnetic properties. Copper is often used in electrical applications due to its excellent conductivity, but it does not retain magnetization. - **B. Aluminum**: - Aluminum is also a non-ferromagnetic material. It has a different electron configuration that does not allow for the alignment of magnetic moments in the same way as iron. While aluminum can be weakly magnetic under certain conditions (paramagnetic), it does not retain magnetization after the removal of an external magnetic field. - **D. Lead**: - Lead is another non-magnetic material. Like copper and aluminum, lead has a full electron configuration that results in no net magnetic moment. It does not exhibit ferromagnetic properties and cannot be magnetized. ### Summary of Key Points: - **Ferromagnetic materials** can retain magnetization after an external magnetic field is removed. - **Iron** is a classic example of a ferromagnetic material due to its unpaired electrons and strong exchange interactions. - **Copper, aluminum, and lead** are non-magnetic materials that do not exhibit ferromagnetism. - Understanding the atomic structure and magnetic properties of materials is crucial for identifying ferromagnetic substances. This thorough understanding of ferromagnetism and the properties of different materials will help you in your studies and exams related to magnetism in physics.