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How do bonding pairs and lone pairs affect molecular shape?

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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.

According to valence-shell electron-pair repulsion (VSEPR) theory, the shape of a molecule is determined by the need to minimize the electrostatic repulsions between electron pairs—both bonding pairs and lone pairs—surrounding a central atom.12

The Role of Electron Pairs

VSEPR theory posits that all electron pairs repel one another and will adopt a geometric arrangement that maximizes their separation. While the overall disposition of these pairs is often determined by their total count, the specific molecular shape is defined by the positions of the atoms attached to the bonding pairs.12

Impact on Bond Angles

The presence of lone pairs influences the final geometry because they exert different repulsive forces than bonding pairs. The hierarchy of repulsion is generally understood as follows:2
* Lone pair–lone pair: Greatest repulsion.
* Lone pair–bonding pair: Intermediate repulsion.
* Bonding pair–bonding pair: Least repulsion.

Because lone pairs are associated with only one nucleus, they occupy more space than bonding pairs, which are constrained between two atoms. Consequently, lone pairs push bonding pairs closer together, causing bond angles to decrease from the ideal geometric arrangement (such as the 109.5° tetrahedral angle). For example, in water and ammonia, the presence of lone pairs results in bond angles smaller than the standard tetrahedral angle.2