What Is a Polar Bond?

A polar bond is a type of covalent chemical bond in which the electromagnetic charge of the molecule is split between both ends; i.e., one end of a molecule has an overall positive charge and the other end has an overall negative charge. The linking of positive and negative charges in separate molecules allows them to bond with one another. The likelihood of atoms to form a polar bond depends on the valence electrons' behavior when interacting with another atom. Valence electrons are those on an atom that can make bonds with other atoms. These types of bonds are essential for the formation of complex molecular structures; charged molecules act as the junctures of more complex compounds.

Covalent bonds form when two atoms meet and have a similar amount of electronegativity, which is a tendency of electrons to be attracted to the valence shell via the nucleus and thus accumulate a negative net charge. The valence shell, or electron shell, is the outer shell of an atom. The property of electronegativity depends partially on the number of electrons in the valence shell, as well as the electrons' distance from the atomic nucleus. A higher number of electrons in the valence shell increases the quantity, whereas distance from the nucleus decreases it. Fluorine is the most electronegative element.

When two atoms meet and one has a higher electronegativity, electrons move to the valence shell of the atom with the lower electronegativity and spend most of the time there, although they may spend brief periods orbiting their original atom. The two atoms become part of a molecule, and the end of the molecule that contains the most electrons on average becomes negatively charged, while the opposite end becomes positively charged.

The simplest example of a polar bond is the water molecule, which consists of an atom with two hydrogens bonded to an atom with one oxygen. A hydrogen atom consists of one proton and one electron, but because the first valence shell can hold two electrons, hydrogen atoms most often bond in pairs and create dihydrogen. Although the two hydrogen atoms are bonded, they can still receive another electron. Meanwhile, oxygen contains eight protons and eight neutrons; the first two electrons occupy the innermost electron orbital shell, leaving six in the valence shell with two slots available. The hydrogen atom and oxygen atom share a pair of electrons between them with the oxygen atom taking the electrons most of the time.

The polar bond in water exists because hydrogen has a lower electronegativity, directly influenced by the number of electrons it tends to draw to itself in a covalent bond: one to oxygen's two. Electrons orbit the oxygen atom and therefore give it a negative charge on that end of the molecule. As a whole, molecules created from a polar bond are electrically neutral, but their charges are concentrated on opposite sides. This is known as a dipole.