Bohr diagrams
Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially-filled valence shells and gain or lose electrons to achieve a stable electron configuration.
Source
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"OpenStax College, Atoms, Isotopes, Ions, and Molecules: The Building Blocks. October 16, 2013."
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OpenStax CNX
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