Electronegativity is a measure of how strongly an atom attracts electrons to itself in a chemical bond. Bond polarity is a property of a covalent bond that describes how equally or unequally the electrons are shared between the two atoms. In this article, we will explore how these two concepts are related and how they affect the properties of molecules.
Contents
What is Electronegativity?
Electronegativity is a unitless number that ranges from 0.7 to 4.0 on the Pauling scale, which is the most commonly used scale for measuring electronegativity. The higher the electronegativity of an atom, the more it tends to pull electrons towards itself in a bond. The electronegativity of an atom depends on several factors, such as the number of protons in the nucleus, the size of the atom, and the configuration of the electrons.
The most electronegative element is fluorine, with an electronegativity of 4.0, while the least electronegative element is francium, with an electronegativity of 0.7. Generally, electronegativity increases from left to right and from bottom to top across the periodic table, as shown in Figure 1.
Figure 1: Electronegativities of the Elements
What is Bond Polarity?
Bond polarity is a property of a covalent bond that describes how equally or unequally the electrons are shared between the two atoms. A covalent bond is formed when two atoms share one or more pairs of electrons. However, not all covalent bonds are created equal. Depending on the difference in electronegativity between the two atoms, the covalent bond can be classified as nonpolar or polar.
A nonpolar covalent bond is one in which the electrons are shared equally between the two atoms. This happens when the two atoms have the same or very similar electronegativities. For example, in a chlorine molecule (Cl2), both chlorine atoms have an electronegativity of 3.0, so they share the pair of electrons equally. A nonpolar covalent bond has no charge separation or dipole moment.
A polar covalent bond is one in which one atom has a greater attraction for the electrons than the other atom. This happens when the two atoms have different electronegativities. For example, in a water molecule (H2O), oxygen has an electronegativity of 3.5, while hydrogen has an electronegativity of 2.1. Therefore, oxygen pulls the electrons more strongly than hydrogen, creating a partial negative charge on oxygen and a partial positive charge on hydrogen. A polar covalent bond has a charge separation or dipole moment.
The degree of polarity of a covalent bond can be estimated by calculating the difference in electronegativity between the two atoms, as shown in Table 1.
Table 1: Electronegativity Difference and Bond Polarity
| Electronegativity Difference | Bond Type |
| 0 | Nonpolar covalent |
| 0–0.4 | Slightly polar covalent |
| 0.5–2.1 | Definitely polar covalent |
| >2.1 | Likely ionic |
An ionic bond is not a true covalent bond, but rather an extreme case of polarity where one atom completely transfers one or more electrons to another atom, forming positive and negative ions that are held together by electrostatic attraction. For example, in sodium chloride (NaCl), sodium has an electronegativity of 0.9, while chlorine has an electronegativity of 3.0. Therefore, sodium gives up its valence electron to chlorine, forming Na+ and Cl- ions.
How Does Bond Polarity Affect Molecular Properties?
Bond polarity affects various properties of molecules, such as shape, polarity, solubility, melting point, boiling point, and reactivity.
- Shape: The shape of a molecule depends on how the atoms are arranged around a central atom and how they repel each other due to their electron pairs. Bond polarity affects the shape of a molecule by creating regions of high and low electron density that influence the repulsion between electron pairs.
- Polarity: The polarity of a molecule depends on how the bond dipoles add up to form a net dipole moment for the whole molecule. Bond polarity affects the polarity of a molecule by creating partial charges on different atoms that may or may not cancel out depending on their orientation.
- Solubility: The solubility of a substance depends on how well it can interact with the solvent molecules through intermolecular forces such as hydrogen bonding, dipole-dipole attraction, and dispersion forces. Bond polarity affects the solubility of a substance by determining its ability to form intermolecular forces with the solvent. Generally, polar substances dissolve in polar solvents, while nonpolar substances dissolve in nonpolar solvents.
- Melting point and boiling point: The melting point and boiling point of a substance depend on how strong the intermolecular forces are between its molecules. Bond polarity affects the melting point and boiling point of a substance by influencing the strength of the intermolecular forces. Generally, polar substances have higher melting points and boiling points than nonpolar substances, because they have stronger intermolecular forces.
- Reactivity: The reactivity of a substance depends on how easily it can undergo chemical reactions with other substances. Bond polarity affects the reactivity of a substance by affecting its stability and tendency to donate or accept electrons. Generally, polar substances are more reactive than nonpolar substances, because they have more charge separation and more potential to form new bonds.
Conclusion
Bond polarity is a property of a covalent bond that describes how equally or unequally the electrons are shared between the two atoms. Electronegativity is a measure of how strongly an atom attracts electrons to itself in a chemical bond. The difference in electronegativity between the two atoms determines the degree of polarity of the covalent bond. Bond polarity affects various properties of molecules, such as shape, polarity, solubility, melting point, boiling point, and reactivity.
