Paramagnetism
Some substances, like iron and water, are influenced by electric and magnetic fields. Iron, of course, forms magnets; water is attracted by static electricity. You can try this by charging a rubber balloon (by rubbing it on wool, for instance) and holding it near a water stream; the water will bend toward the balloon. This occurs because water is a polar molecule (it has a dipole), and the static charge on the balloon pulls the water to it.
Two categories of magnetism have been created. Most substances, of course, are not attracted to magnets; they are called diamagnetic. Those substances that are responsive to magnets are paramagnetic. Paramagnetism results when an orbital is left with an unpaired electron; the more unpaired orbitals, the most responsive a substance is to a magnet. For example, helium is diamagnetic because its 1s orbital is filled (1s [+-]). Lithium, however, has the electron configuration 1s [+-] 2s [+ ]. In this case, the 2s orbital has an unpaired electron, making lithium paramagnetic. Chromium is an extreme example: remember that it departs from the normal filling order to half-fill its 3d orbitals, which results in a more stable configuration. Its six unpaired electrons ([Ar] 3d [+ ][+ ][+ ][+ ][+ ] 4s [+ ]) make it highly paramagnetic.
Most paramagnetic substances are still too "unmagnetized" to be noticed in ordinary conditions; it takes a very powerful magnet to induce a response. However, some substances have atoms in which the unfilled orbitals align, producing a much stronger type of magnetic response called ferromagnetism. Most atoms in a solid will rapidly vibrate out of alignment, so only special solids (such as iron, cobalt, and nickel) maintain their orientation at room temperature. Above a certain temperature (called the Curie temperature), however, these substances will lose their magnetism as well.
After all the new theories we've covered in this chapter, you'd think that we have explained every possible nuance of orbitals and bonding. However, paramagnetism (of all things) messed up our picture of the universe once again. What's the problem, you ask? Oxygen, good old O2, with all of its 1s and 2sp3 hybrid orbitals filled, is paramagnetic! True enough--liquid oxygen will cling to the poles of a powerful magnet. How can this be? To explain this thorn in our sides, a new theory had to be developed, called the molecular-orbital (MO) theory.
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