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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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