Bond polarity

What is bond polarity?

Bond polarity can be defined as the difference in electronegativityEN) between the two bonding atoms. The greater the difference in electronegativity, the more polar the bond.

Types of bonds

If a bond is formed by two equal atoms (same electronegativity) it gives rise to a covalent bond, they share electrons equally and the bond is purely covalent or non-polar.

For example, the bromine molecule (Br2), composed of two bromine atoms (which necessarily have identical electronegativities), has a covalent bond in which electrons are shared equally.

fig-1

If there is a large electronegativity difference between the two atoms in a bond, as is usually the case between a metal and a nonmetal, the electron from the metal is almost completely transferred to the nonmetal, and a bond is ionic.

For example, potassium and chlorine (KCl) form an ionic bond.

fig-2

In the case where there is an intermediate electronegativity difference between the two atoms, as occurs between two different nonmetals, the bond that originates is polar covalent.

For example, HCl has a polar covalent bond.

fig-3

Although all attempts to label the continuum of bond polarity into particular bond types are necessarily arbitrary, it is useful from a chemistry point of view to classify bonds as covalent, polar covalent and ionic, based on the difference in electronegativity between the bonding atoms, as shown in the Table and Figure below.

fig-4

In addition, the bond polarity can be quantified by the size of its dipole moment.

Examples