Module 4

• Atomic theory


  The theory that all matter is composed of tiny, indivisible particles called atoms
• Historical development of atomic theory

  1. Ancient philosophers

  2. Dalton's atomic theory
• Democritus


  • Proposed the concept of indivisible particles called "atomos" or atoms that constitute all matter
  • Believed that cutting a piece of wood into smaller parts would eventually reach a point where it could not be divided further, illustrating the concept of indivisible atoms
• Dalton's atomic theory


  • Elements are composed of indivisible atoms
  • Atoms of the same element are identical in mass and properties
  • Compounds form when atoms combine in specific ratios
  • Chemical reactions involve the rearrangement of atoms
• Water (H₂O)


  • Consists of hydrogen and oxygen atoms combined in a specific ratio, supporting Dalton's postulate of compound formation
• Atomic structure and subatomic particles

  1. Discovery of electrons

  2. Rutherford's gold foil experiment

  3. Bohr's atomic model
• J.J. Thomson


  • Experimented with cathode rays and discovered electrons, proposing the "plum pudding" model of the atom
  • Cathode ray tube experiments demonstrated the deflection of negatively charged particles (electrons) by electric and magnetic fields
• Rutherford's gold foil experiment

  • Alpha particles were expected to pass through the gold foil; however, some particles were deflected, suggesting a dense, positively charged nucleus
• Bohr's atomic model


  • Proposed a quantum model of the atom with electron orbits or energy levels
  • Electrons occupy specific energy levels or shells around the nucleus, transitioning between levels by absorbing or emitting energy
• Modern atomic theory and quantum mechanics

  1. Quantum mechanics

  2. Atomic orbitals and electron configuration
• Quantum mechanics


  • Schrodinger, Heisenberg, and others developed quantum mechanics to describe the behavior of electrons in atoms, introducing concepts such as wave-particle duality and quantum numbers
  • The electron cloud model represents the probability of finding an electron in a particular region around the nucleus, incorporating wave-like properties
• Atomic orbitals and electron configuration

  • Pauli Exclusion Principle, Hund's Rule, and Aufbau Principle define the arrangement of electrons in atomic orbitals based on quantum numbers
• Electron configuration of carbon (C)

  • 1s² 2s² 2p²
• Applications and implications of atomic theory

  • Chemical reactions and bonding
  • Technological and scientific advancements
• Covalent bonding


  • Involves the sharing of electrons between atoms, determining molecular structures and properties
• Nuclear power plants


  • Utilize fission reactions to generate electricity, highlighting the practical applications of atomic theory
• Atomic number (Z)


  Represents the number of protons in the nucleus of an atom, defining its identity as a specific element
• Carbon (C)


  • Has an atomic number of 6, indicating it has 6 protons
• Mass number (A)


  Represents the total number of protons and neutrons in the nucleus of an atom
• Carbon-12


  • Has a mass number of 12, indicating it has 6 protons and 6 neutrons
• Protons (P)


  Are equal to the atomic number (Z) of the element, representing the number of positively charged particles in the nucleus
• Sodium (Na)


  • With atomic number 11 has 11 protons
• Electrons (E)


  Are equal to the number of protons in a neutral atom, balancing the positive charge of protons
• Chlorine (Cl)


  • With atomic number 17 has 17 electrons in a neutral state
• Neutrons (N)


  Are calculated by subtracting the atomic number (Z) from the mass number (A), representing the neutral particles in the nucleus
• Nitrogen-14 (^14N)


  • Has 7 protons and 7 neutrons, deducing 7 neutrons by subtracting the atomic number 7 from the mass number 14