Chemical polarity

The polarity of a water molecule, showing the partial negative charge on the oxygen atom and the partial positive charges on the hydrogen atoms.

Boron trifluoride (BF₃) is a nonpolar molecule despite having polar bonds, due to its symmetrical shape.

A diagram showing the dipole moment of a water molecule.

The polarity of an ammonia molecule, with a lone pair of electrons on the nitrogen atom contributing to its dipole moment.

Chemical polarity is a concept in chemistry that describes the distribution of electric charge around atoms, molecules, or chemical groups. Polarity arises from the differences in electronegativity between atoms bonded together, leading to an uneven distribution of electron density. This results in a molecule having a partial positive charge on one side and a partial negative charge on the other, creating a dipole moment.

Molecular Polarity

Molecular polarity is determined by both the polarity of individual bonds and the shape of the molecule. A molecule can have polar bonds but still be nonpolar overall if the molecular geometry allows for the dipoles to cancel each other out.

Polar Bonds

A bond is considered polar if there is a significant difference in electronegativity between the two atoms involved. For example, in a water molecule (H₂O), the oxygen atom is more electronegative than the hydrogen atoms, resulting in polar O-H bonds.

Nonpolar Bonds

Nonpolar bonds occur when the electronegativity difference between the bonded atoms is negligible, such as in diatomic molecules like N₂ or O₂, where the atoms share electrons equally.

Molecular Geometry

The shape of a molecule plays a crucial role in determining its overall polarity. For instance, carbon dioxide (CO₂) has polar bonds, but its linear shape causes the dipoles to cancel out, making it nonpolar. In contrast, water has a bent shape, resulting in a net dipole moment.

Effects of Polarity

Chemical polarity has significant effects on the physical and chemical properties of substances, including:

• Solubility: Polar molecules tend to dissolve well in polar solvents (like water), while nonpolar molecules dissolve better in nonpolar solvents (like hexane).
• Boiling and Melting Points: Polar substances generally have higher boiling and melting points due to stronger intermolecular forces such as hydrogen bonding.
• Reactivity: Polarity can influence the reactivity of molecules, as polar molecules are more likely to participate in dipole-dipole interactions and hydrogen bonding.

Applications

Understanding chemical polarity is essential in fields such as biochemistry, pharmacology, and materials science. It helps in predicting the behavior of molecules in biological systems, designing drugs, and developing new materials with specific properties.

Also see

• Electronegativity
• Intermolecular forces
• Hydrogen bond
• Solubility
• Molecular geometry