Overview
The lecture explains hydrogen bonding in water and other molecules, focusing on the causes and significance of partial charges (dipoles) and their impact on molecule interactions.
Water Molecule Structure and Bonds
- Water (H₂O) consists of two hydrogen atoms covalently bonded to one oxygen atom.
- Each hydrogen shares one electron with oxygen, giving oxygen a full outer shell of eight electrons.
- Four electrons on the oxygen are not shared and remain as lone pairs.
Unequal Electron Distribution and Partial Charges
- Oxygen end of water has a surplus of electrons (lone pairs), while hydrogen ends have fewer electrons.
- This creates an unequal electron distribution across the water molecule.
- Oxygen gains a partially negative charge (δ⁻), and hydrogen gains a partially positive charge (δ⁺).
- These are weak partial charges, not full ionic charges.
- Water is called a dipole because it has two poles (positive and negative regions).
Hydrogen Bonding
- Partially positive hydrogen of one water molecule can attract the partially negative oxygen of another, forming a hydrogen bond (shown as a dotted line).
- Oxygen atoms in water can form two hydrogen bonds due to their size, while hydrogen atoms can form only one.
- Hydrogen bonds are important in many biological molecules, including cellulose.
Hydrogen Bonds in Other Molecules
- Molecules with –OH (hydroxyl), –NH (amino), and –CO (carbonyl) groups can also form hydrogen bonds.
- In –OH and –NH groups, hydrogen is partially positive; in –CO, oxygen is partially negative.
- Hydrogen bonds are crucial for the interaction of biomolecules such as amino acids and lipids.
Key Terms & Definitions
- Covalent Bond — a chemical bond formed by sharing electron pairs between atoms.
- Lone Pair — electrons on an atom not involved in bonding.
- Partial Charge (δ⁺/δ⁻) — a slight electrical charge due to unequal electron distribution.
- Dipole — a molecule with two poles of charge, one positive and one negative.
- Hydrogen Bond — a weak bond between a partially positive hydrogen and a partially negative atom like oxygen or nitrogen.
Action Items / Next Steps
- Review how hydrogen bonding affects the properties of water and biomolecules.
- Prepare for upcoming lessons on amino acids and lipids, focusing on hydrogen bonding roles.