A valence electron belongs to the electronic arrangement used to describe a chemical atom's chemical bonds and ordinary chemical reactions. For the neutral hydrogen, carbon, nitrogen, and oxygen chemical atoms used here, the valence electrons occupy the outermost occupied electron shell. Their respective electron configurations are , , , and , giving one, four, five, and six valence electrons. See Chemistry 2e, 6.4: electron configurations and valence electrons.
Lewis structures show these valence electrons while omitting the inner Electrons. Consequently, eight drawn valence electrons in water do not mean eight total Electrons: neutral water has ten, with two inner oxygen Electrons omitted. Neutral ammonia likewise has ten total Electrons but eight valence electrons; neutral carbon dioxide has twenty-two total Electrons but sixteen valence electrons.
This distinction makes the small counts useful: connections and possible lone pairs can be organized without representing every Electron or solving the entire many-Electron wavefunction. The outermost-electron shell shortcut applies to the examples above, rather than serving as a universal definition for all chemical elements; the electronic arrangement relevant to chemical bonds can also involve other electron subshells.
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Valence electrons are the electrons in the outermost shell (or energy level) of an atom that are involved in chemical bonding and reactions. These electrons are crucial because they determine how an atom interacts with other atoms, influencing the formation of bonds in molecules and compounds. The number of valence electrons varies among different elements and can be determined by the group number in the periodic table. For example, elements in Group 1 have one valence electron, while those in Group 17 have seven.