Molecules interact with magnetic and electric fields and with electromagnetic radiation. These interactions allow us to identify molecules, determine their concentration, and deduce their structure. It provides the possibility of qualitative as well as quantitative analysis.
Some molecules possess a magnetic moment comparable to that of an electromagnet.
Normally the molecular moments are oriented at random, owning to the thermal motion of the molecules. Paramagnetism and ferromagnetism result from a partial alignment of these moments in the direction of an applied magnetic field. Diamagnetism results from an interaction between all the molecular electrons and an applied field, which induces small moments directed in opposition to the field. The total moment per unit volume in the direction of the applied field as directly proportional to the field strength. The proportionality constant is called the magnetic susceptibility per unit volume.