Magnetism
Magnetism has intrigued humankind for millennia. Because magnetic and magnetically susceptible objects affect each other at a distance, without being in contact with each other, the effect appears to be magical. An understanding of magnetic effects that are much more subtle than the orientation of a compass needle by Earth's magnetic field has contributed greatly to our current concepts of chemical bonding and molecular structure.
Naturally magnetized pieces of magnetite, FeO, have been known for thousands of years and for the past several hundred years sailors and explorers have used the effect of Earth's magnetic field on an iron needle to help them maintain their direction. The great advance in the understanding of magnetism that has been exploited in chemistry occurred only in the last century when physicists James Clerk Maxwell and others determined relationships that link electricity with magnetism. When Gilbert Newton Lewis convincingly demonstrated the importance of electrons in chemical bonding and Irwin Schrödinger developed a mathematical model for the interactions of electrons in atoms that could be expanded to include a description of the magnetic properties of electrons in molecules, the stage was set for using experimentally measured magnetic properties to deduce information about the chemical structure of materials.
Moving electric charge creates a magnetic field. If the distribution of moving electric charge is not symmetric, the magnetic field will also be unsymmetric. In some materials, such as the natural magnetlodestone, the unsymmetric magnetic fields of individual atoms are aligned in thesame direction within the bulk material. If this condition persists even in the absence of an external magnetic field, the material is a permanent magnet. Magnetite that does not act as a magnet may not have been exposed for a sufficient time or under other appropriate conditions to become magnetized by Earth's magnetic field. Placing it close enough to a magnet or placing it in a strong electromagnetic field can magnetize nonmagnetized magnetite. Similarly, metallic iron and many other metals, alloys, oxides, sulfides and other compounds containing iron and a number of other transition metals can be magnetized. Materials that can become magnetized are composed of atoms whose electrons can be predominately oriented asymmetrically and remain oriented asymmetrically once the external magnetic field is removed. The asymmetric electron distribution in such materials reinforces itself. Such materials are called ferromagnetic (like iron).
Most materials are not ferromagnetic, but there are a variety of magnetic effects that other materials exhibit. In most materials, the electrons of the atoms are all paired with each other. The molecules of most substances in non- crystalline bulk materials are randomly oriented in liquids and solids. A bulk sample of a randomly oriented, spin-paired material will not be attracted into a magnetic field. In fact, such materials are very slightly (compared to the magnitude of ferromagnetic effects) repelled by a magnetic field. These materials are said to be diamagnetic. Although diamagnetic effects are usually very small, one class of materials shows strikingly large diamagnetism: superconductors. Superconducting materials are so strongly repelled by magnetic fields that they are levitated by them. There have been proposals to utilize this property of superconductors (the Meissner effect) to construct levitated trains that would experience nearly zero friction by traveling above magnetized tracks.
Some materials, particularly compounds of the transition metals but also some compounds of nitrogen and other elements with an odd number of valence electrons, have unpaired electrons. The magnetic moments resulting from the unpaired electrons often align themselves with an external magnetic field, drawing the materials into the magnetic field. This effect can be so great for compounds with atoms that have several unpaired electrons, such as those of manganese (II) with five unpaired electrons per manganese (II) atom, that a sample can weigh 10% more in the field of a laboratory magnet than outside the field. Compounds that are attracted into a magnetic field are paramagnetic. The gain in weight at a particular magnetic field strength (the paramagnetic susceptibiliy) usually decreased inversely with the temperature on the absolute (Kelvin) scale, a phenomenon known as Curie's law.
By observing such factors as the degree to which a material becomes magnetized, the weight gained or lost in a magnetic field, the temperature dependence of such changes, and the dependence of such changes on the magnitude of the external magnetic field, chemists have determined that there are a number of different patterns that can be correlated with the interactions of electrons of one atom with neighboring atoms in a molecular or ionic structure or with the overall repeating structure of atoms in a crystalline structure. For example, in some structures, the magnetic moments of entire molecules within a crystal are aligned in opposite directions, a condition termed antiferromagnetism. If the magnetic moments of atoms within the molecules are aligned in opposite directions, the condition is antiferrimagnetism. Sometimes the magnetic moment in one direction is larger than that in the other and the material shows some degree of magnetization at high enough magnetic fields. These materials are called parasitic ferromagnets or parasitic ferrimagnets, respectively. The magnetic properties of some crystalline substances change drastically when they are subjected to pressure along one direction of the crystal. These substances are called piezomagnets, analogous to the terminology for piezoelectric crystals whose electrical conductivity changes when they are subjected to pressure.
The behavior of substances in a magnetic field may be used to deduce the number of unpaired electrons present in atoms of the substance, the ways that unpaired electrons on one atom interact with those on a nearby atom, and to deduce short-and long-range structural effects in crystalline compounds. Our understanding of magnetism has progressed from the mysterious to fundamental science useful in solving modern chemical questions about the structure of matter.
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