Atomic Models and Radioactivity

• AS 91172 v2 (2.5) Atomic and Nuclear Physics is an internal assessment worth 3 credits, evaluated through a test with resubmission and retesting options.
• The topic covers models of the atom, radioactivity, and nuclear change.
• Key special characters used include alpha (𝛼), beta (𝛽), gamma (ɣ), and beta position (𝛽+).

Models of the Atom

• Dalton's Model (1803): All matter is Atoms, the smallest indivisible unit, with each element having its own unique atom and no concept of charges.
• Thomson's Model (1897): Atoms contain electrons (-) and must have positives (+), making atoms divisible. Electrons can be removed.
• Thomson's Plum Pudding Model: Electrons are "raisins" embedded in a positively charged "batter".

Rutherford's Gold Foil Experiment (1908-1911)

• Aim: To detail Thompson’s atom
• Equipment included an evacuated chamber, thin gold foil (50 atoms thick), an alpha particle source (Uranium-238), and a ZnS screen.

Observations

• Most 𝛼 particles passed through the gold foil without deviation (99.99%).
• A small number of 𝛼 particles were slightly deflected (0.009%).
• A minuscule number of 𝛼 particles were significantly deflected, some completely backwards (0.001%).

Conclusions

• Atoms are mostly empty space.
• The mass is concentrated in a tiny, dense, central nucleus.
• The nucleus is positively charged.
• Electrons orbit the nucleus.

Atomic Information

• An atom is mostly empty space. The vast majority of alpha particles passed through the gold foil as if nothing was there.
• Matter is found in an incredibly dense nucleus, which reverses the momentum of alpha particles.
• Nucleus' positive charge repels positive alpha particles electrostatically.
• Electrons orbit the nucleus due to electrostatic attraction, preventing them from being sucked into the nucleus.
• The nucleus is very small, as only a tiny number of alpha particles interact with it.
• Isotopes are variants of an element with the same number of protons but different numbers of neutrons.
  • Carbon-12: 6 protons, 6 neutrons.
  • Carbon-14: 6 protons, 8 neutrons.
  • Carbon-13: 6 protons, 7 neutrons.
  • Carbon-15: 7 protons, 8 neutrons.

Nuclear Stability and Radioactive Decay

• Isotopes: Variants of an element with the same number of protons but varying numbers of neutrons.
• Nuclear Stability: Determined by the neutron-to-proton ratio. Neutrons provide the strong force to counter electrostatic repulsion between protons.
• Stable isotopes are common, while unstable isotopes undergo radioactive decay.
• Alpha (𝛼) particles are helium nuclei (He42).
  • Example: U92238 → He24 + Th90234
• Beta (𝛽) particles are electrons emitted from the nucleus (e-10). Rasium-230 (Ra88230) emits a beta particle to becomes actinium-230.

Gamma and Beta Positron Decay

• Gamma (ɣ) Decay: Excited nuclei release energy by emitting gamma photons.
  • B511* → ɣ00 + B511
• Antimatter:
  • Electron (-), Proton (+), Neutron (0)
  • Positron (+) e1+0, Antiproton (-), Antineutron (0)
• Beta Positron Decay:
  • Example: Carbon-11: C611 → e1+0 + B511*
• PET Scan: Uses glucose with O-15, a beta positron emitter. The positron annihilates with an electron, emitting gamma photons detectable for rapid glucose metabolism (e.g., tumors).

Characteristics of 𝛼, 𝛽, and ɣ Particles

• Alpha (𝛼) Emitters: Produce helium gas as 𝛼 particles (He2+) steal electrons from the air, turning into He gas atoms.
  • Example: Pu94238 → He24 + U92234
• Beta (𝛽) Emitters: Daughter nucleus releases energy to become stable.
  • Example: U92239 → e-10 + Np93239 , Np93239 → e-10 + Pu94239
• Nuclear Change: Rutherford bombarded N-14 with 𝛼 particles, producing O-17 and a proton (H11).
  • N714 + He24 → O817 + H11

Background Radiation and Uses for Radioactivity

• Background Radiation: Comes from stone, concrete, the sun, human activity.
• Smoke Alarms: Use Americium-241 to detect smoke; alarms trigger when smoke disrupts electric charge.

Half-Life

• Half-life is unique to each isotope.
• Radioactive decay is random; half-life is the time for halving of:
  • Undecayed mass
  • Number of undecayed nuclei
  • Rate of emission
• Example: Graphins half-life = 8 days

Half-Life Graphs and Calculations

• Iodine-123 (I-123) for thyroid cancer treatment with a half-life of 13 hours.

Mass-Energy Transformations

• In nuclear reactions, mass changes.
  • mProducts > mReactants: energy absorbed
  • mProducts < mReactants: energy released
• Example: Plutonium-238 decay:
  • Pu94238 → He24 + U92234
  • Mass difference (Δm) is calculated: ΔE = Δmc2, where c = 3.00×108ms-1
  • Mass loss converts to energy release.

Nuclear Change Equations

• Nuclear Fission: A large nucleus splits into smaller nuclei.
  • Example: U92235 + n01 → U92236 → Xe54140 + Sr3894 + 2n01
• Nuclear Fusion: Two smaller nuclei combine into a larger one.
  • Example: H12 + H13 → He24 + n01

Energy Released From Nuclear Fusion of H-2 and H-3

• Fusion of H-2 and H-3: Releases more energy per kg of reactants than fission of U-235.
• Calculations:
  • ΔE from 1 U-235 fission is 2.9628×10-11J.
  • ΔE from fusion of 1 H-2 with 1 H-3 is 3.0024×10-12J.
• The number of nuclei in 1kg is calculated to determine total energy release.

Explore atomic models from Dalton to Rutherford, including the gold foil experiment. Understand radioactivity, nuclear change, and special characters like alpha, beta, and gamma particles.