**Correct Option: B. 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 occurs due to the alignment of magnetic moments (tiny magnetic fields produced by the electrons) within the material. **Why is Iron a Ferromagnet?** 1. **Atomic Structure**: Iron has unpaired electrons in its d-orbitals. These unpaired electrons contribute to a net magnetic moment, which is crucial for ferromagnetism. 2. **Exchange Interaction**: In iron, the exchange interaction between neighboring atoms leads to a parallel alignment of their magnetic moments. This alignment is strong enough to overcome thermal agitation, allowing the material to retain its magnetization. 3. **Curie Temperature**: Iron has a specific temperature known as the Curie temperature (around 770°C) above which it loses its ferromagnetic properties. Below this temperature, iron can be magnetized and will retain that magnetization when the external field is removed. ### Why the Other Options are Incorrect **A. Copper** - **Non-Magnetic**: Copper is a diamagnetic material, meaning it does not have unpaired electrons that can contribute to magnetization. When exposed to a magnetic field, copper will not retain any magnetization once the field is removed. **C. Aluminum** - **Paramagnetic**: Aluminum is a paramagnetic material, which means it has some unpaired electrons, but the magnetic moments do not align in a way that allows for permanent magnetization. When the external magnetic field is removed, the magnetic moments return to a random orientation, and thus aluminum does not retain magnetization. **D. Bismuth** - **Diamagnetic**: Bismuth is also a diamagnetic material. It has no unpaired electrons and does not exhibit any permanent magnetization. Like copper, it will not retain any magnetization after the external magnetic field is removed. ### 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. - **Copper and Bismuth**: Both are diamagnetic and do not retain magnetization. - **Aluminum**: Paramagnetic but does not retain magnetization after the external field is removed. ### 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. - Copper and bismuth are diamagnetic, while aluminum is paramagnetic, neither of which can retain magnetization. - Understanding the properties of materials is crucial for identifying ferromagnets.