Overview
This lecture explains electronegativity and its role in creating polar and nonpolar covalent bonds, focusing on examples in biological molecules.
Electronegativity and Covalent Bonds
- Electronegativity is an atom's tendency to attract shared electrons in a covalent bond.
- Oxygen and nitrogen are highly electronegative and often do not share electrons equally.
- Unequal sharing of electrons creates polar covalent bonds; equal sharing forms nonpolar covalent bonds.
Polar Covalent Bonds
- Polar covalent bonds occur when one atom attracts electrons more strongly, creating partial charges.
- "Polar" means there are two different ends or poles, like a magnet.
- Water (H₂O) is a classic example, with oxygen being partially negative (δ-) and hydrogens partially positive (δ+).
- The shape and properties of water depend on its polar covalent bonds, making water molecules "sticky" to each other.
Common Polar Covalent Bonds in Biology
- Four major polar covalent bonds involve highly electronegative atoms: O-H, O-C, N-H, and N-C.
- Oxygen forms polar bonds with hydrogen and carbon (O-H, O-C).
- Nitrogen forms polar bonds with hydrogen and carbon (N-H, N-C).
- In these bonds, oxygen or nitrogen is partially negative, while the other atom is partially positive.
- Polar molecules tend to mix well with water ("like dissolves like").
Nonpolar Covalent Bonds
- Nonpolar covalent bonds involve equal sharing of electrons between atoms.
- Bonds between identical atoms (O=O, H-H) are always nonpolar.
- Carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds are also typically nonpolar.
- Nonpolar molecules are hydrophobic ("water-fearing") and don't mix well with water, like fats.
Key Terms & Definitions
- Electronegativity — An atom's ability to attract electrons in a bond.
- Polar Covalent Bond — A bond where electrons are shared unequally, creating partial charges.
- Nonpolar Covalent Bond — A bond with equal electron sharing and no charge difference.
- Partial Charge (δ+, δ-) — A slight electrical charge from unequal electron sharing; delta (δ) denotes "partial".
- Hydrophobic — Substances that do not mix well with water.
Action Items / Next Steps
- Be able to identify polar and nonpolar covalent bonds in molecules.
- Memorize the four major polar covalent bonds (O-H, O-C, N-H, N-C).
- Review how water’s polarity affects its interactions in biological systems.