Definition of a Lone Pair
A lone pair is a pair of valence electrons in an atom that are not shared with another atom in a covalent bond. Instead, these electrons are exclusively associated with one atom, playing a critical role in determining a molecule's shape and chemical properties.
How Lone Pairs Influence Molecular Geometry
According to VSEPR (Valence Shell Electron Pair Repulsion) theory, lone pairs occupy more space than bonding pairs and exert stronger repulsive forces. This increased repulsion pushes bonding pairs closer together, causing distortions in the ideal geometric shapes and affecting the molecule's overall three-dimensional structure.
A Practical Example: Water (H₂O)
In a water molecule (H₂O), the central oxygen atom forms two covalent bonds with hydrogen atoms and possesses two lone pairs of electrons. The strong repulsion from these two lone pairs on the oxygen atom forces the H-O-H bond angle to be approximately 104.5°, resulting in a bent molecular shape.
Importance in Chemical Reactions
Lone pairs are significant in chemical reactions because they represent available electron density that can be donated to form new bonds. For example, in Lewis acid-base chemistry, a molecule or ion with a lone pair acts as a Lewis base, readily donating this electron pair to a Lewis acid.