Nuclear Chemistry Quiz

Chapter 30: Nuclear Physics and Radioactivity

• Contents: Structure and properties of the nucleus, Binding Energy and Nuclear Forces, Radioactivity (Alpha, Beta, Gamma decay), Conservation of Nucleon Number and Other Conservation Laws, Half-Life and Rate of Decay, Calculations Involving Decay Rates and Half-Life, Decay Series, Radioactive Dating, Stability and Tunneling, Detection of Particles.

Structure and Properties of the Nucleus

• Atoms are categorized into groups based on similar chemical properties, which suggests they're composed of simpler building blocks.
• Experiments using particle probes indicated that atoms have a dense, positively charged nucleus and a cloud of negative electrons.
• The nucleus is made of protons and neutrons (collectively called nucleons).
• Protons have a positive charge and a mass of 1.67262 × 10⁻²⁷ kg.
• Neutrons are electrically neutral and slightly more massive than protons, with a mass of 1.67493 × 10⁻²⁷ kg.
• Nuclei are referred to as nuclides.
• Atomic number (Z) = number of protons.
• Atomic mass number (A) = total number of nucleons (protons + neutrons).
• Neutron number (N) = A - Z
• Nuclides are symbolized using the chemical symbol (X), atomic number (Z), and atomic mass number (A) (e.g., ¹⁴₆C).
• Nuclides with the same atomic number (Z) but different neutron numbers (N) are called isotopes.
• Natural abundance is the percentage of a particular isotope in nature.
• The size of an atomic nucleus is somewhat fuzzy due to wave-particle duality; high-energy electron scattering measurements provide size estimations.
• The mass of atoms is measured relative to the carbon-12 atom.
• Atomic mass units (u): 1 u = 1.6605 × 10⁻²⁷ kg = 931.5 MeV/c².
• Electrons are significantly less massive than nucleons.

Binding Energy and Nuclear Forces

• The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons.
• The "missing" mass is converted into energy (binding energy) during the nucleus formation.
• Binding energy per nucleon is a measure of how tightly bound the nucleus is; higher binding energies indicate greater stability. More massive nuclei require extra neutrons to overcome the electrostatic repulsion between protons.
• Stable nuclei generally do not contain protons with a large number greater than Z = 82
• The strong nuclear force binds nucleons together; it's very strong but short-range, effectively zero when nucleons are more than approximately 10⁻¹⁵ m apart.
• The Coulomb force (electrostatic force) is also present, leading to repulsion between protons.
• Extra neutrons are needed to overcome the repulsion of the larger nuclei.
• The weak nuclear force governs many nuclear decays.

Radioactivity

• Radioactivity is the disintegration or decay of unstable nuclei.
• Radioactive rays (alpha, beta, and gamma rays) are emitted during decay.
  • Alpha rays: helium nuclei, barely penetrate paper.
  • Beta rays: electrons, penetrate 3mm aluminum.
  • Gamma rays: electromagnetic radiation, penetrate several centimeters of lead.
• Alpha and beta particles are deflected by magnetic fields in opposite directions; gamma rays are not deflected.

Alpha Decay

• Alpha decay occurs when the strong nuclear force cannot hold a large nucleus together.
• The decay produces a smaller, more stable nucleus and an alpha particle.
• The difference between the mass of the parent nucleus and the sum of the daughter nucleus and alpha particle mass is the disintegration energy/

Beta Decay

• Beta decay occurs when a nucleus emits an electron (β⁻) or a positron (β⁺)
• The weak nuclear force is responsible for this decay type.
• A neutron decays into a proton, electron; and an anti-neutrino.
• The need for a neutrino was discovered due to inconsistencies in energy and momentum conservation in beta decay.

Gamma Decay

• Gamma decay is the emission of a high-energy photon.
• It is usually followed by another type of decay when the nucleus is in an excited state.
• A and Z remain the same, so the chemical element remains the same.

Conservation Laws

• Electric charge, linear momentum, angular momentum, mass-energy, and nucleon number are conserved in all radioactive decays.
• The total number of nucleons remains constant during decay.

Half-Life and Rate of Decay

• Nuclear decay is a random process, independent of previous decays.
• Decay rate is proportional to the number of nuclei present and the decay constant.
• The half-life is the time it takes for half of the nuclei to decay.

Radioactive Dating

• Radioactive dating utilizes the decay of isotopes like carbon-14 to determine the age of organic materials.
• The ratio of carbon-14 to carbon-12 in the atmosphere has remained roughly constant.
• Living organisms have a consistent carbon-14/carbon-12 ratio.
• After death, the carbon-14 decays, allowing the determination of the age of a specimen. For dating materials older than 60,000 years, other isotopes are more suitable.

Stability and Tunneling

• Nuclei decay through alpha emission (releasing energy), but not immediately due to energy barrier.
• The alpha particle can escape by tunneling through this energy barrier—a quantum mechanical phenomenon governed by the Heisenberg uncertainty principle.
• The wider the energy barrier, the slower the rate of decay.

Detection of Particles

• Individual particles, like electrons, neutrons, and protons, are not directly visible.
• Instruments like Geiger counters, scintillation counters, cloud chambers, and wire drift chambers are used to detect these particles. These devices detect the presence of particles and measure other characteristic properties.