A lone pair references a pair of electrons in the valence shell of an atom which are not bonded to another atom or molecule. Since only those electrons in the valence, or outer, shell of an atom participate in bonding, lone pairs are studied in chemistry to account for the different shapes of molecules with the same number of bonds. Since electrons repel each other, molecules that have central atoms with a lone pair will be shaped differently than those that do not.
Electrons orbit the nucleus of an atom in various shells. Each shell can hold a certain number of atoms, and electrons always orbit in pairs, spinning in opposite directions. The electrons in the outer shell of the atom, called the valence shell, may be shared with other atoms in order to form bonds and create molecules. In some molecules, all the valence shell electrons in the central atom are bonded to another atom, but in others only some are bonded. A pair of electrons in an atom that are not bonded to another atom is called a lone pair.
In chemistry, loan pairs are studied because they affect the way certain molecules are shaped, which in turn may affect how the molecules behave. Explained by the valence shell electron pair repulsion (VSEPR) theory, electrons naturally repel one another, which accounts for the shapes of various molecules. For example, beryllium chloride (BrCl2) consists of a beryllium atom bonded to two chlorine atoms. Each chlorine atom is attached to the beryllium atom by a pair of beryllium's electrons through covalent bonding. Since no unbonded electrons remain in the valence shell, the farthest apart the electrons holding the chlorine atoms can move from each other is 180°, creating a linear molecule.
Tin chloride (SnCl2), however, does have a loan pair of electrons. Just like beryllium chloride, tin chloride has two chlorine atoms bonded to a tin atom by electron pairs. Unlike beryllium chloride, tin chloride has an additional unbonded pair of electrons, a lone pair, in the valence shell as well. This results in the tin chloride molecule having a bent shape as all three electron pairs try to move a maximum distance from each other.
The resulting angle between the chloride atoms should then be 120°. Scientific research has discovered it is actually 95°, however. This discrepancy is a result of the loan pair's orbital. The orbital of a loan pair is greater than the orbital of a bonded pair, which leads to the angle between atoms being smaller.