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How does VSEPR theory predict the shapes of covalently bonded molecules?

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Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction. — covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.

Valence-shell electron-pair repulsion (VSEPR) theory predicts the shapes of covalently bonded molecules by focusing on the role of electron pairs surrounding a central atom. The theory is based on the principle that all electron pairs—both bonding pairs and lone pairs—repel one another, and molecules adopt a geometric arrangement that minimizes these electrostatic repulsions.21

To predict a molecular shape, the following procedure is typically used:
* Identify Electron Pairs: Determine the total number of electron pairs around the central atom, disregarding the distinction between bonding and lone pairs.1
* Determine Arrangement: Allow these pairs to position themselves around the central atom to maximize their mutual separation (e.g., four pairs adopt a tetrahedral disposition).1
* Define Molecular Geometry: Identify the positions of the attached atoms based on the electron pair arrangement. The final shape is determined by the disposition of the atoms, not the electron pairs themselves.1

To account for variations in bond angles, VSEPR theory assumes that repulsion is greatest between lone pairs, less between lone pairs and bonding pairs, and least between bonding pairs. While useful, the model has limitations, such as failing to predict exact bond lengths or the magnitude of bond angle decreases.13