What is a Lone Pair?
Lone pair is a set of electrons present in an atom’s valence shell that did not participate in any bond-formation reaction. Since they refuse to bond with the other atoms, they are also called the non-bonding electrons. While drawing the molecules’ structure, the lone pair electrons on shown as dots (..) above the atom.
An atom can have one, two, and three pairs of non-bonding electrons. The Nitrogen group carries one lone pair; the Oxygen group has two, and the Halogens have three lone pairs.
How Lone pairs are formed?
An atom’s electrons are present in the extranuclear region and are roughly divided into core electrons (closest to the nucleus) and valence electrons (outermost and away from the nucleus). The valence electrons are the ones involved in bonding reactions.
For example, the Oxygen atom has 8 electrons divided into two levels. The lowest level has two electrons closer to the nucleus (the core electrons), and the highest level has six electrons, the valence electrons. The six electrons participate in bonding reactions for the Oxygen atom.
Suppose the Oxygen atom reacts with the Hydrogen atom (with only one electron) to form one water molecule. In that case, Oxygen only uses its two electrons to form two single covalent bonds.
The remaining four electrons are unused and are the non-bonding electron lone pair of the Oxygen atom.
Another method to find the atom’s lone pair is to draw Lewis structures. Lewis structures describe the bonding between atoms to form molecules.
For example, a Chlorine atom has 17 electrons divided into three levels as- 2, 8, and 7. The outermost level with 7 electrons is the Chlorine atom’s valence electrons. It needs one more electron in its valence shell so that Chlorine attains the stability of the nearest inert element Argon (Argon- 2,8, 8 electrons).
When two Chlorine atoms form a two-electron covalent bond, the two electrons become part of both atoms' nuclei. Both Chlorine atoms thus attain their octet stability.
Drawing the Lewis structure of the Chlorine molecule (Cl2) reveals the two-electron bond, and the remaining three pairs are the non-bonding/lone pair electrons.
However, not all nonbonding pairs of electrons are lone pairs. For example, in transition metals with plenty of valence electrons, most d-electrons do not take part in bond formation.
These d-electrons are nevertheless involved in the transition metals exhibiting other magnetism properties and influence the formation of coordination compounds with other ligands. Examples of co-ordination compounds of d-block elements are- K4Fe(CN)6, Cu(NH3)4SO4, etc. (transition metals are marked in blue and ligands in black)
The lone pair electrons are not responsible for holding atoms together in a bond but influence other physical and chemical properties of -shape, H-bonding, polarity, dipole moment, and nucleophilicity.
