3. Lithium Through Neon   Previous PageNext Page
     Filled Shells and the Rule of Eight: A Summary

The theme of this chapter has been the special stability of a closed inner shell of two electrons, and of a filled second shell of eight electrons. Atoms such as lithium, which have only a few electrons outside the inner shell, will lose them easily and show a low electronegativity. In contrast, O and F, which lack only one or two electrons of completing the second shell, will attract electrons and show a high electronegativity. Electronegativity describes in relative terms how bonded atoms will compete for electrons.

Covalent and ionic bonds are extreme types, with most actual bonds being intermediate in character. Whether a bond is described as mainly covalent or mainly ionic depends on whether the electron pair of the bond is shared more or less equally between atoms, or is pulled sharply toward one of them. When like atoms are bonded, whether the bond is unlocalized and metallic (as in Li, as discussed previously), or localized as an electron pair shared between two particular atoms, depends on how tightly the atoms normally hold electrons.

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