1. Introduction

• Living organisms seem different from ordinary chemical systems, but they follow chemical and physical laws.
• Life’s chemistry is special because:
  1. Carbon-based: Focuses on organic chemistry.
  2. Aqueous environment: Most reactions occur in water.
  3. Complexity: Even simple cells have highly intricate chemistry.
  4. Polymers 聚合体: Built from large molecules made of smaller subunits.
  5. Regulation: Cellular chemical reactions are tightly controlled.

2. Chemical Bonds

• Elements: Substances like C, H, O, N form the basis of living matter.
• Atoms: Smallest units of elements; consist of:
  • Protons (+) 质子 and neutrons 中子 in the nucleus.
  • Electrons (-) 电子 in surrounding orbitals.
• Molecules: Atoms join via chemical bonds to form molecules.

3. Types of Atoms in Cells

• Living organisms use relatively few types of atoms.
• Four main elements (C, H, N, O) make up ~96% of living matter.
• Other important elements: Na, Mg, P, S, Cl, K, Ca.

4. Atomic Structure & Properties

• Atomic number: Number of protons; defines element identity.
• Isotopes 同位素: Atoms with same protons but different neutrons; some are radioactive.
• Atomic weight: Approx. mass of protons + neutrons.
• Avogadro’s number: $6 \times 10^{23}$ atoms/molecules per mole.