= Molecular geometry
{wiki}
Why do <methane>, <ammonia>, and <water> have different shapes when each central <chemical atom> is surrounded by eight <valence electrons>? The answer separates the positions of <atomic nuclei> from the <electron domains> used in a <VSEPR model>.
<Molecular geometry> describes the three-dimensional arrangement of the <atomic nuclei> in a <molecule>, including <bond lengths> and <bond angles>. For a larger <molecule>, a shape name often describes just the neighbours of one central <chemical atom>. A <Lewis structure> records connections and electron bookkeeping; its flat layout does not fix <molecular geometry>.
In the <VSEPR model>, <electron pair geometry> includes both bonding <electron domains> and <lone pairs>, whereas <molecular geometry> counts only the <chemical atoms>. In <methane>, four <bond pairs> and no <lone pairs> give <tetrahedral molecular geometry>. In <ammonia>, three <bond pairs> and one <lone pair> give <trigonal pyramidal molecular geometry>. In <water>, two <bond pairs> and two <lone pairs> give <bent molecular geometry>. All three use the same ideal tetrahedral <electron pair geometry>; this does not make their actual <bond angles> equal. See https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity[Flowers, Theopold, Langley and Robinson, Chemistry 2e, §7.6: electron-pair and molecular geometries].
\Image[/molecular-geometry-domains-and-atoms.png]
{title=Four <electron domains>, three arrangements of <atomic nuclei>: <methane>, <ammonia>, and <water>}
{id=image-molecular-geometry-domains-and-atoms}
{description=Original geometric constructions. The upper row is an ideal <VSEPR model> counting scaffold: orange LP markers identify <lone pairs>, not additional <chemical atoms> or measured <electron density>. The lower row shows only <chemical atoms>; <ammonia> and <water> use the sourced approximate <bond angles> discussed below. Sizes and <bond lengths> are arbitrary. Three-dimensional projections can change the apparent <bond angles> on the page. Dotted outlines are geometric guides, not extra <chemical bonds>.}
A static drawing also needs a physical interpretation. The <atomic nuclei> undergo <oscillations>; an arrangement at a minimum of <potential energy> and an experimentally averaged arrangement need not have identical <bond lengths> or <bond angles>. This distinction matters when comparing measurements with <quantum mechanics> calculations; see the https://goldbook.iupac.org/terms/view/BT07003[IUPAC Gold Book: experimental bond-length conventions]. <Gas electron diffraction> gives a concrete route from a laboratory signal to <molecular geometry> rather than inferring shape from a drawing alone.
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