What is the role of lone pairs in covalent bonding?

Lone pairs in covalent bonding are unshared pairs of electrons that influence the shape and reactivity of a molecule.

In more detail, covalent bonding involves the sharing of electron pairs between atoms. These shared pairs of electrons, known as bonding pairs, are what hold the atoms together in a molecule. However, not all electrons in an atom are necessarily involved in bonding. Those that are not shared are known as lone pairs.

Lone pairs play a crucial role in determining the shape of a molecule. According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, electron pairs will arrange themselves to be as far apart as possible to minimise repulsion. This includes both bonding pairs and lone pairs. As lone pairs occupy more space than bonding pairs, they cause greater repulsion and hence have a greater effect on the shape of the molecule.

For example, in a water molecule (H2O), the oxygen atom has two lone pairs and two bonding pairs. These four pairs of electrons arrange themselves in a tetrahedral shape around the oxygen atom, but because the lone pairs occupy more space, the shape of the water molecule is bent or V-shaped, not tetrahedral.

Lone pairs also play a role in the reactivity of a molecule. They can be donated to form a dative covalent (or coordinate) bond, where both electrons in the bond come from one atom. This is often the case when a molecule reacts with a proton (H+ ion), as the proton has no electrons of its own.

In summary, lone pairs in covalent bonding are unshared pairs of electrons that not only influence the shape of a molecule, but also its reactivity. Understanding their role is key to understanding many aspects of chemistry.