= Valence electron
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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 $1s^1$, $1s^2\,2s^2\,2p^2$, $1s^2\,2s^2\,2p^3$, and $1s^2\,2s^2\,2p^4$, giving one, four, five, and six <valence electrons>. See https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations[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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