Why should you care which way the electrons are spinning? Spin determines how many electrons can occupy an orbital, and also, which orbitals electrons will fill first within a subshell. Each electron in an atom is uniquely labelled by its set of 4 quantum numbers. (This is the Pauli Exclusion Principle, for which Wolfgang Pauli received a Nobel Prize in 1945.) Three quantum numbers (n, l, and ml) are needed to describe the orbital the electron occupies. Since there are two possible values for the fourth quantum number, you may place no more than two electrons in an orbital when filling in the orbital diagram. You must also ensure that when there are two electrons in the same orbital, they have opposite spins. Electrons repel each other, and tend to align their spins whenever possible. When filling up a subshell on an orbital diagram, then, you must spread the electrons among different orbitals to keep them as far apart as possible, and line up their spins. (This is called Hund's rule).
The discovery that electrons and other particles have a 'spin' property had profound consequences in physics and chemistry. Technological applications include magnetic resonance imaging (MRI), which allows detailed mapping of soft tissues in the body, mainly for diagnosis and monitoring of diseases like cancer. Future applications will include 'quantum computers' which will exploit spin states to store information on an atomic size scale.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/electrons/faq/print-why-arrows-for-electrons.shtml