1. Covalent Bonds and Molecules

• Molecule: A cluster of atoms held together by covalent bonds (共价键) where electrons are shared, not transferred.
• Covalent bond: Atoms share electrons to complete their outer shells, stabilizing the molecule.
• Example: H₂ molecule — two hydrogen atoms share their electrons to achieve a filled shell.
• The balance of attractive and repulsive forces (排斥力) between nuclei keeps the molecule stable.

2. Periodic Table and Atomic Properties

• Atomic number determines the vertical position in the periodic table; atoms in the same column behave similarly.
• Some elements tend to give away electrons to form ionic bonds (离子键) with atoms like chlorine (氯).
• Life is dominated by light elements:
  • C, H, O, N make up ~99% of atoms in the human body.
  • Others like Na, Mg, P, S, Cl, K, Ca are trace but essential.

3. Covalent Bond Geometry

• Bond length (键长): Distance between nuclei when covalent bonds form (e.g. H₂ = 0.074 nm).
• Bond angles (键角) and geometry (几何结构) define the 3D shape:
  • Water (H₂O): V-shape, angle ~109°.
  • Carbon forms tetrahedral geometry (四面体结构) with four covalent bonds.
  • Nitrogen and oxygen have predictable geometries too.

4. Single, Double, and Multiple Bonds

• Single bond (单键): One shared electron pair.
• Double bond (双键): Two shared pairs; shorter and stronger; less rotational freedom.
• Example:
  • Ethane 乙烷: C–C single bond allows free rotation.
  • Ethene 乙烯: C=C double bond is rigid and planar.

5. Polar vs. Nonpolar Covalent Bonds