Atomic Models and Nuclear Radiation

Jul 12, 2024

Atomic Models and Nuclear Radiation

Timeline of Atomic Models

John Dalton's Atomic Model (1803)

  • Matter is made of small indivisible atoms.
  • Atoms cannot be subdivided, created, or destroyed.
  • Atoms of the same element have the same properties.
  • Atoms of different elements have different properties.
  • Atoms of different elements can form compounds.

Thomson's Plum Pudding Model (1897)

  • Discovery of the electron.
  • Atom is electrically neutral; positive and negative charges are equal.
  • Atom consists of a sphere of positive charge with negatively charged electrons embedded in it (like raisins in a pudding).
  • Thomson won a Nobel Prize for his discovery of the electron.

Rutherford's Nuclear Model (1909)

  • Hans Geiger and Ernest Marsden's experiment under Rutherford's supervision.
  • Most of the atom is empty space.
  • Mass is concentrated in a dense, positively charged nucleus.
  • Electrons orbit the nucleus.
  • Rutherford’s gold foil experiment:
    • Most alpha particles passed straight through the foil.
    • Some were deflected at small angles; a few bounced back.
    • Conclusion: Atom is mostly empty space with a dense nucleus.

Structure of the Nucleus

Rutherford's Nuclear Model (1909)

  • Positively charged nucleus at the center.

Discovery of Protons (1919)

  • Rutherford bombarded nitrogen gas, discovering protons.

Discovery of Neutrons (1932)

  • James Chadwick discovered neutrons within the nucleus.
  • Neutrons have no charge; mass similar to protons.
  • Chadwick won a Nobel Prize for this discovery.

Nucleons

  • Nucleus made of protons (+1 charge) and neutrons (0 charge).
  • Electrons (-1 charge) orbit the nucleus.
  • Mass of proton/neutron is much larger than an electron.
  • Number of protons (atomic number) identifies the element.

Nuclide Notation

  • X: Chemical symbol.
  • A: Mass number (total protons + neutrons).
  • Z: Atomic number (number of protons).
  • Example: Lithium (chemical symbol Li, atomic number 3, mass number 7).

Isotopes

  • Atoms with the same number of protons but different numbers of neutrons.
  • Example: Hydrogen isotopes (Protium, Deuterium, Tritium).
  • Carbon isotopes (C-12, C-13, C-14).
  • Unstable isotopes are radioactive.
  • Sources of background radiation: rocks, soil, air, food, cosmic rays.

Nuclear Fission and Fusion

Nuclear Fission

  • Splitting a large nucleus into smaller nuclei and releasing energy.
  • Example: Uranium-235 absorbs a neutron, splits into Krypton-92 and Barium-141, releasing energy and more neutrons (chain reaction).

Nuclear Fusion

  • Combining two light nuclei to form a heavier nucleus and releasing energy.
  • Example: Fusion in the Sun (Hydrogen isotopes Deuterium and Tritium form Helium-4).
  • Requires extremely high temperatures and pressures.

Radiation and Its Detection

Types of Radiation

  • Alpha (α) particles: 2 protons, 2 neutrons (He nucleus).
  • Beta (β) particles: High-speed electrons.
  • Gamma (γ) rays: Electromagnetic waves.
  • Relative charges: α (+2), β (-1), γ (0).
  • Relative masses: α (heavy), β (1 electron mass), γ (none).

Detection

  • Geiger-Müller (GM) tube measures radiation count rates.
  • Background radiation must be subtracted to get accurate measurements.
  • Experiments with absorbers (paper, aluminum) help identify the type of radiation.

Effects in Electric and Magnetic Fields

  • Alpha particles deflected toward negative plate (positive charge).
  • Beta particles deflected toward positive plate (negative charge).
  • Gamma rays not deflected (no charge).
  • Deflection patterns in magnetic fields explained by Fleming’s left-hand rule.

Radioactive Decay

Decay Processes

  • Alpha Decay: Heavy nucleus emits an alpha particle.
  • Beta Decay: Neutron transforms into a proton and emits a beta particle.
  • Gamma Decay: Nucleus emits gamma rays, reducing its energy but not changing its structure.

Half-life

  • Time for half the nuclei in a sample to decay.
  • Activity measured in becquerels (disintegrations per second).
  • Example: Iodine-131 has an 8-day half-life.

Uses of Radiation

Smoke Detectors

  • Alpha particles ionize air; smoke blocks ionization, triggering an alarm.

Measuring Thickness

  • Beta radiation used to control material thickness in manufacturing.

Medical Applications

  • Gamma tracers diagnose organ issues (gamma camera tracks tracer).
  • Radiotherapy uses gamma rays to target and kill cancer cells.
  • Sterilizing medical equipment and food using gamma radiation.

Safety and Effects of Ionizing Radiation

Health Effects

  • Can damage DNA, causing mutations or cancer.
  • Acute exposure causes burns, immune suppression.

Safety Precautions

  • Lead-lined boxes, handling with gloves/tongs, minimizing exposure time, protective clothing.
  • Proper disposal of radioactive waste (long half-life).

Encouragement to Learn More

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