What phenomenon describes the alignment of electrons when exposed to an external magnetic field?

The alignment of electrons when exposed to an external magnetic field is described by magnetic resonance. This phenomenon occurs when the magnetic moments of electrons respond to an external magnetic field, aligning themselves in a specific orientation. When subjected to the appropriate frequency of electromagnetic radiation, such as radio waves, these aligned electrons can absorb energy and transition to a higher energy state, resulting in resonance.

This principle underlies various applications, including Magnetic Resonance Imaging (MRI) in medical imaging, where the magnetic resonance of hydrogen nuclei in the body provides detailed images. Understanding magnetic resonance is crucial for comprehending how materials interact with magnetic fields, particularly in fields like chemistry and physics, where electron behaviors are pivotal to various interactions and energy states.