DR MARK J. WINTER / SCIENCE PHOTO LIBRARY DR MARK J. WINTER / SCIENCE PHOTO LIBRARY
An electron orbital is a region around an atomic nucleus (not seen) in which one or a pair of electrons is most likely to exist. The orbital is seen halved at right to show axes of symmetry. The 1s orbital is filled in every element except hydrogen, in which it contains just one electron. This configuration is formally written 1s1. In the next element, helium, the 1s orbital is full, containing two electrons. This is written 1s2.
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