Bond and Molecule Polarity: Study Notes for General Chemistry

Bond and Molecule Polarity

Introduction to Bond Polarity

Bond polarity is a fundamental concept in chemistry that describes the distribution of electrical charge between atoms in a chemical bond. It arises due to differences in electronegativity between bonded atoms, leading to unequal sharing of electrons.

• Nonpolar Covalent Bond: Electrons are shared equally between atoms of similar electronegativity. Example: Cl2.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (δ+ and δ−) on the atoms. Example: HCl.

• Ionic Bond: Electrons are transferred completely from one atom to another, creating ions. Example: NaCl.

Key Concept: A polar covalent bond acts as a dipole, with a slightly positive pole and a slightly negative pole.

Electronegativity and Bond Polarity

Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. The difference in electronegativity between two atoms determines the bond type and its polarity.

• Partial Charges: The atom with higher electronegativity acquires a partial negative charge (δ−), while the other becomes partially positive (δ+).

• Bond Dipole: Represented by an arrow pointing toward the more electronegative atom.

Examples of Polar Covalent Bonds:

• C—O (carbon-oxygen)

• N—S (nitrogen-sulfur)

• O—S (oxygen-sulfur)

Additional info: The larger the electronegativity difference, the more polar the bond.

Polar Bonds and Polar Molecules

A molecule is polar if it contains polar bonds arranged asymmetrically, resulting in a net dipole moment. The overall molecular polarity depends on both the bond polarities and the molecular geometry.

• Bond Dipole: Created when atoms with different electronegativities share electrons unequally.

• Molecular Dipole: The vector sum of all bond dipoles in a molecule.

• Example: Water (H2O) is bent, so the two O—H bond dipoles do not cancel, resulting in a polar molecule with a permanent dipole.

Key Point: For a molecule to be polar overall, it must contain polar bonds that do not offset one another.

Polar Bonds and Nonpolar Molecules

Some molecules contain polar bonds but are nonpolar overall due to their symmetrical geometry, which causes the bond dipoles to cancel each other out.

• Bond Dipole: Exists between atoms of different electronegativities.

• Nonpolar Molecule: The bond dipoles are arranged symmetrically and offset one another, resulting in balanced electron density.

• Example: Carbon dioxide (CO2) is linear, so the two C=O bond dipoles cancel, making the molecule nonpolar.

Molecular Polarity and Its Effects

The polarity of a molecule is related to the overall charge separation and affects its physical and chemical properties, such as solubility, boiling point, and interaction with electric fields.

• Charge Separation: The degree of polarity is determined by how much the molecule aligns with an external electric field.

• Example: Hydrogen fluoride (HF) aligns with an electric field due to its molecular dipole.

• Physical Properties: Polar molecules tend to have higher boiling points and are more soluble in polar solvents like water.

Dipole Moments

The dipole moment is a quantitative measure of molecular polarity, expressed in Debye units (D). It depends on both the magnitude of the charge separation and the distance between charges.

• Formula:

$\mu = q \times r$

• $\mu$ = dipole moment (Debye, D)

• $q$ = magnitude of the partial charges

• $r$ = distance between the charges

Comparison of Dipole Moments and Properties

Dipole moments can be compared among molecules to predict their relative polarity and physical properties.

Molecule	Dipole Moment (D)	Boiling Point (°C)	Solubility in Water
CH3OCH3	1.08	-24	Slightly soluble
CH3CH2OH	1.69	78	Very soluble

Additional info: Molecules with higher dipole moments are generally more polar, have higher boiling points, and are more soluble in polar solvents.