BackCovalent Bonds, Electronegativity, and Molecular Polarity
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Covalent Bonds and Molecular Polarity
Types of Covalent Bonds
Covalent bonds involve the sharing of electrons between atoms. The nature of this sharing determines whether the bond is polar or nonpolar.
Polar Covalent Bonds: Electrons are shared unequally due to a difference in electronegativity between the atoms. This creates partial charges (δ+ and δ−) on the atoms.
Nonpolar Covalent Bonds: Electrons are shared equally because the atoms have equal or nearly equal electronegativity. The molecule is electrically neutral overall.
Electronegativity
Electronegativity is the tendency of an atom to attract electrons towards itself in a chemical bond. The greater the electronegativity, the more strongly an atom pulls electrons.
Electronegativity differences between atoms determine bond polarity.
Common scale: Fluorine is the most electronegative element.
Partial Charges and Bond Polarity
When a bond is polar, one atom becomes partially negative (δ−) and the other partially positive (δ+). For example, in a hydrogen chloride (HCl) molecule, chlorine is δ− and hydrogen is δ+.
These partial charges are indicated as δ+ (partial positive) and δ− (partial negative).
Polar covalent bonds result in a dipole moment.
Electronegativity Difference and Bond Type
The difference in electronegativity values between two atoms can be used to predict the type of bond:
Electronegativity Difference | Bond Type |
|---|---|
0 | Nonpolar Covalent |
0.5 or less | Mostly Covalent |
0.5 - 1.9 | Polar Covalent |
2.0 or more | Ionic |
Polarity increases with an electronegativity difference of at least 0.5.
Geometric Asymmetry and Molecular Polarity
For a molecule to be polar, it must have both polar bonds and a geometric asymmetry that prevents the dipoles from canceling out. If the molecule is symmetrical, the dipoles may cancel, resulting in a nonpolar molecule even if it contains polar bonds.
Example: Carbon dioxide (CO2) has polar bonds but is linear and symmetrical, so it is nonpolar overall.
Example: Water (H2O) has polar bonds and a bent shape, making it a polar molecule.
