What is polarity of molecules?

Short Answer

The polarity of molecules refers to the unequal distribution of electric charge within a molecule. A molecule becomes polar when one part of it carries a slight negative charge and another part carries a slight positive charge. This happens when atoms in the molecule have different electronegativities.

Polar molecules, like water (H₂O), have an uneven shape or unequal electron sharing, while non-polar molecules, like oxygen (O₂), share electrons equally. Polarity affects many properties such as solubility, boiling point, melting point, and how molecules interact with each other.

Detailed Explanation :

Polarity of Molecules

The polarity of molecules is an important concept in chemistry that helps explain how molecules behave, interact, and react with each other. Molecular polarity is mainly determined by the arrangement of atoms, the shape of the molecule, and the difference in electronegativity between bonded atoms. Polarity influences many physical and chemical properties such as solubility, boiling points, melting points, and intermolecular forces.

Understanding polarity helps us know why oil and water do not mix, why water dissolves many substances, and why certain molecules have strong attractions while others do not. It also explains many biological processes, such as how enzymes bind to substrates and how nutrients move in the body.

Meaning of Polarity of Molecules

Polarity of molecules can be defined as:

“The uneven distribution of electric charge within a molecule due to unequal sharing of electrons between atoms.”

When one atom attracts shared electrons more strongly, it becomes slightly negative (δ⁻), while the other atom becomes slightly positive (δ⁺). This results in a dipole, which is a pair of opposite charges within a molecule.

Two Main Conditions for Molecular Polarity

A molecule becomes polar only if:

  1. The bonds are polar (electronegativity difference exists between atoms).
  2. The molecular geometry is not symmetrical, so dipoles do not cancel out.

Both conditions must be met for a molecule to be polar.

Role of Electronegativity in Polarity

Electronegativity difference between atoms determines whether the bond is:

  • Non-polar covalent → equal sharing of electrons
  • Polar covalent → unequal sharing of electrons
  • Ionic → electron transfer

For example:

  • H–Cl bond is polar because Cl attracts electrons more strongly.
  • H–H bond is non-polar because both atoms attract electrons equally.

Thus, polarity often begins with differences in electronegativity.

Molecular Geometry and Polarity

Even if a molecule contains polar bonds, its shape determines whether the molecule is overall polar.

  • If the shape is asymmetrical, dipoles do not cancel → molecule is polar.
  • If the shape is symmetrical, dipoles cancel → molecule becomes non-polar.

Examples:

  • H₂O (water) → bent shape → polar
  • CO₂ (carbon dioxide) → linear and symmetrical → non-polar
  • CH₄ (methane) → tetrahedral and symmetrical → non-polar
  • NH₃ (ammonia) → pyramidal shape → polar

Thus, polarity depends on both electronegativity and geometry.

Examples of Polar and Non-Polar Molecules

Polar Molecules

  • Water (H₂O)
  • Hydrogen chloride (HCl)
  • Ammonia (NH₃)
  • Sulphur dioxide (SO₂)
  • Ethanol (C₂H₅OH)

Characteristics:

  • They have partial positive and negative ends.
  • They dissolve in water.
  • They have high boiling and melting points.

Non-Polar Molecules

  • Oxygen (O₂)
  • Nitrogen (N₂)
  • Methane (CH₄)
  • Carbon dioxide (CO₂)
  • Benzene (C₆H₆)

Characteristics:

  • No charge separation.
  • Do not dissolve in water.
  • Have low boiling and melting points.

Dipole Moment

Polarity of a molecule is measured using dipole moment, which shows the strength of the polarity.

  • Higher dipole moment → more polar molecule
  • Zero dipole moment → non-polar molecule

Dipole moment depends on charge difference and distance between atoms.

Effects of Polarity on Physical Properties

Polarity affects many physical characteristics:

  1. Solubility
  • Polar molecules dissolve in polar solvents (e.g., water).
  • Non-polar molecules dissolve in non-polar solvents (e.g., oil).

This leads to the rule “like dissolves like”.

  1. Boiling and Melting Points

Polar molecules attract each other strongly → higher boiling point.
Non-polar molecules have weaker forces → lower boiling point.

  1. Intermolecular Forces

Polar molecules have strong dipole-dipole attractions and hydrogen bonds.
Non-polar molecules have weak dispersion forces.

  1. Behaviour in Electric Fields

Polar molecules align themselves in an electric field because of their positive and negative ends.

Importance of Polarity in Chemistry and Biology

Polarity plays a major role in:

  • Solubility of substances in the body
  • Formation of cell membranes
  • DNA structure
  • Enzyme-substrate interactions
  • Drug design
  • Behaviour of gases and liquids
  • Separation techniques like chromatography

Without polarity, many biological and chemical processes would not occur.

Everyday Examples of Polarity

  • Water dissolves sugar and salt because it is polar.
  • Oil floats on water because it is non-polar.
  • Soap works because it has both polar and non-polar parts.
  • Water’s polarity gives it high surface tension and boiling point.

These examples show how polarity affects daily life.

Conclusion

The polarity of molecules refers to the uneven distribution of electrical charge in a molecule due to unequal electron sharing and an unsymmetrical shape. Molecules with polar bonds and asymmetrical structures are polar, while those with symmetrical shapes are non-polar. Polarity affects solubility, boiling point, melting point, and intermolecular forces. It plays a crucial role in chemistry, biology, and everyday processes such as dissolving, bonding, and molecular interactions.