Covalent bonds result from atoms sharing electrons, which holds them together in molecules. While the term “sharing” suggests equality, the electrons are often not shared equally between the two bonded atoms. The degree to which this shared electron pair is pulled toward one atom over the other determines whether the bond is classified as nonpolar or polar. This mechanism allows both atoms to achieve a more stable electron configuration.
Defining Covalent Bonds and Electronegativity
This type of chemical bond is most common between two nonmetal atoms. The key to understanding unequal sharing is electronegativity, which measures an atom’s tendency to attract a shared pair of electrons toward itself.
Electronegativity values are generally measured using the Pauling scale, where Fluorine is the most electronegative element (4.0). Elements positioned toward the upper right of the periodic table exhibit higher electronegativity because their smaller size and greater nuclear charge allow them to attract bonding electrons more strongly.
The Role of Electronegativity Difference
The polarity of a covalent bond is determined by the numerical difference between the electronegativity values of the two bonded atoms. When two atoms with identical electronegativity values bond, the difference is zero, resulting in a perfectly nonpolar covalent bond where the electrons are shared equally. This occurs typically when two atoms of the same element bond, such as in \(\text{O}_2\) or \(\text{Cl}_2\).
A nonpolar covalent bond exists when the electronegativity difference is very small, generally falling between 0.0 and 0.4 on the Pauling scale. As the difference increases, the bond becomes polar covalent, spanning a difference of approximately 0.4 to 1.7. Within this range, the shared electrons spend more time orbiting the nucleus of the more electronegative atom.
This unequal distribution creates a separation of charge within the bond, known as a bond dipole moment. The more electronegative atom acquires a partial negative charge (\(\delta^-\)). Conversely, the less electronegative atom is left with a partial positive charge (\(\delta^+\)) because the electron pair is pulled away from its nucleus.
If the electronegativity difference becomes very large, typically greater than 1.7, the bond character shifts. The pull of the more electronegative atom is so strong that the electrons are effectively transferred, rather than shared. This complete transfer results in the formation of full positive and negative ions, defining the bond as predominantly ionic instead of covalent.
How to Identify Polar and Nonpolar Bonds
Identifying the type of bond requires looking up the electronegativity values for the two atoms and calculating the difference. A bond between two carbon atoms (\(\text{C-C}\)) has a difference of zero, making it a classic example of a nonpolar covalent bond.
When hydrogen bonds to chlorine, a polar covalent bond (\(\text{H-Cl}\)) is created. Chlorine has a greater electron-attracting power than hydrogen, so the shared electrons are drawn closer to the chlorine atom. This leaves the hydrogen atom partially positive (\(\delta^+\)) and the chlorine atom partially negative (\(\delta^-\)), establishing the electrical poles of the bond.
Another common example is the bond between oxygen and hydrogen (\(\text{O-H}\)), such as those found in a water molecule. Oxygen is more electronegative than hydrogen, pulling the shared electrons strongly toward itself. This unequal sharing gives the oxygen atom a partial negative charge and the hydrogen atoms a partial positive charge, demonstrating a clear case of polarity.