Editing Atom (section)

=== Magnetic moment ===
{{Main|Electron magnetic moment|Nuclear magnetic moment}}

Elementary particles possess an intrinsic quantum mechanical property known as [[Spin (physics)|spin]]. This is analogous to the [[angular momentum]] of an object that is spinning around its [[center of mass]], although strictly speaking these particles are believed to be point-like and cannot be said to be rotating. Spin is measured in units of the reduced [[Planck constant]] (ħ), with electrons, protons and neutrons all having spin {{frac|1|2}}&nbsp;ħ, or "spin-{{frac|1|2}}". In an atom, electrons in motion around the [[Atomic nucleus|nucleus]] possess orbital [[angular momentum]] in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.<ref name=hornak2006 />

The [[magnetic field]] produced by an atom—its [[magnetic moment]]—is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field, but the most dominant contribution comes from electron spin. Due to the nature of electrons to obey the [[Pauli exclusion principle]], in which no two electrons may be found in the same [[quantum state]], bound electrons pair up with each other, with one member of each pair in a spin up state and the other in the opposite, spin down state. Thus these spins cancel each other out, reducing the total magnetic dipole moment to zero in some atoms with even number of electrons.<ref name=schroeder2 />

In [[Ferromagnetism|ferromagnetic]] elements such as iron, cobalt and nickel, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a spontaneous process known as an [[exchange interaction]]. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. [[Paramagnetism|Paramagnetic materials]] have atoms with magnetic moments that line up in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field.<ref name=schroeder2 /><ref name=goebel20070901 />

The nucleus of an atom will have no spin when it has even numbers of both neutrons and protons, but for other cases of odd numbers, the nucleus may have a spin. Normally nuclei with spin are aligned in random directions because of [[thermal equilibrium]], but for certain elements (such as [[xenon|xenon-129]]) it is possible to [[spin polarization|polarize]] a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called [[hyperpolarization (physics)|hyperpolarization]]. This has important applications in [[magnetic resonance imaging]].<ref name=yarris1997 /><ref>{{cite book
|last1=Liang|first1=Z.-P.|last2=Haacke|first2=E.M.
|editor=Webster, J.G.|year=1999
|volume=2
|title=Encyclopedia of Electrical and Electronics Engineering: Magnetic Resonance Imaging
|publisher=John Wiley & Sons
|isbn=978-0-471-13946-1|pages=412–426}}</ref>