VSEPR theory

VSEPR theory predicts molecular shape by treating electron domains around a central atom as mutually repelling regions that arrange to minimize repulsion.

Electron domain: A region of electron density around an atom, such as a bond or lone pair. VSEPR counts these regions because each contributes to the central atom’s arrangement.
Lewis structure: A diagram showing valence electrons as bonds and lone pairs in a molecule or ion. A Lewis structure supplies the electron-domain count VSEPR uses.
Methane: The simplest alkane, with formula CH₄ and a carbon atom bonded to four hydrogens. Four equivalent bonding domains give methane its tetrahedral shape.
Valence shell electron pair repulsion model: A qualitative model that predicts molecular shape from repulsions among electron domains around a central atom. This expanded name describes the same model; the acronym is the usual shorthand.
Lone pair: A pair of valence electrons localized on one atom rather than shared in a bond. Lone pairs occupy space and often compress nearby bond angles more strongly than bonding domains.
Valence electron: An electron in an atom’s outer shell that can participate in bonding or remain nonbonding. Counting valence electrons is the starting point for assigning bonds and lone pairs.
Ammonia: A nitrogen hydride with formula NH₃ and one lone pair on nitrogen. Its lone pair changes the tetrahedral domain arrangement into a trigonal-pyramidal molecular shape.
Valence bond theory: A bonding model that describes covalent bonds through overlap of atomic orbitals. It accounts for bonding through orbital overlap rather than relying on domain repulsions alone.
Bond angle: The angle between two bonds that share an atom. Predicted repulsions explain why measured bond angles depart from ideal geometries.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. VSEPR is a way to infer this arrangement from electron-domain repulsions.

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