Atomic Models: Thomson & Rutherford

Atomic Models

• Thomson's Model (Plum Pudding Model): An atom is a sphere of positive charge with negatively charged electrons embedded within. It resembles a watermelon with positive charge as the red part and electrons as seeds.
• Rutherford's Model (Nuclear Model): A more accurate atomic model, where most of the atom's mass and positive charge is concentrated in a tiny, dense nucleus. Electrons orbit the nucleus in circular paths.
  • Rutherford's Alpha Scattering Experiment: Bombarding a thin gold foil with alpha particles revealed:
    • Most particles passed straight through, indicating mostly empty space.
    • Some particles were deflected at small angles, suggesting a positively charged region.
    • A few particles were strongly deflected or bounced back, revealing a dense, positively charged nucleus.
• Bohr's Model: This model proposes stationary orbits, where electrons revolve in specific, quantized paths without radiating energy.
  • Key Principles of Bohr's Model:
    • Stationary Orbits: Electrons orbit the nucleus in specific circular orbits.
    • Quantized Energy: Electrons in these orbits possess specific, discrete energy levels.
    • Energy Absorption and Emission: Electrons can change orbits by absorbing or emitting energy in the form of light.
  • Limitations of Bohr's Model: It fails to explain fine structure splitting and the wave nature of electrons.

Atomic Structure

• Composition of the Nucleus: The nucleus contains protons (positive charge) and neutrons (neutral charge).
• Atomic Number (Z): The number of protons in an atom's nucleus, defining the element.
• Mass Number (A): The total number of protons and neutrons in the nucleus.
• Atomic Mass Unit (amu): 1/12th the mass of a carbon-12 atom (approximately 1.66053906660 × 10^-27 kg).
• Nuclear Radius: The nucleus's radius is roughly proportional to the cube root of the mass number.

Isotopes, Isomers, and Isobars

• Isotopes: Atoms of the same element with the same atomic number (Z) but different neutron numbers (different mass numbers, A). Examples include Carbon-12 and Carbon-14.
• Isobars: Atoms of different elements with different atomic numbers (Z) but the same mass number (A). Examples include Potassium-40 and Argon-40.
• Isomers: Atoms of the same element with the same atomic number and mass number but different energy states.

Mass-Energy Equivalence and Binding Energy

• Einstein's Mass-Energy Equivalence: E=mc² describes the relationship between mass and energy, implying mass can be converted to energy and vice-versa.
• Binding Energy: The energy required to separate all nucleons from a nucleus. Higher binding energy indicates greater nuclear stability.
• Binding Energy per nucleon: Binding energy divided by mass number, representing the average energy per nucleon needed to separate it from the nucleus.

Nuclear Forces

• Nuclear Force: A short-range, attractive force holding nucleons in the nucleus. It's much stronger than the electrostatic repulsion between protons.

Nuclear Reactions

• Nuclear Reactions: Reactions involving changes in atomic nuclei, often releasing or absorbing energy.
• Q-Value: The energy released or absorbed in a nuclear reaction, calculated by comparing the mass of reactants and products.
  • Positive Q-value: Energy-releasing (exothermic) reaction.
  • Negative Q-value: Energy-absorbing (endothermic) reaction.

Nuclear Fission and Nuclear Fusion

• Nuclear Fission: Splitting a heavy nucleus into two lighter nuclei, releasing a large amount of energy. Example: Uranium-235 fission.
• Nuclear Fusion: Combining two light nuclei to form a heavier nucleus, releasing even more energy than fission. Example: Hydrogen isotopes fusing into helium.

Applications of Nuclear Physics

• Nuclear Power: Fission used to generate electricity in power plants.
• Medical Imaging: Radioisotopes used in techniques like PET scans.
• Radiation Therapy: Radioisotopes used to target and destroy cancerous cells.
• Carbon Dating: Radioactive carbon-14 used to determine the age of artifacts and fossils.
• Nuclear Weapons: Fission and fusion reactions are used in nuclear weapons.

Explore atomic models, including Thomson's plum pudding model and Rutherford's nuclear model. Learn about the alpha scattering experiment and how it revealed the structure of the atom.